Much like painting with a brush or drawing with a pencil, there are physical limitations to the amount of detail we can pack into an area. We can do this infinitely digitally, with zooming in and vectors. But physically, even the smallest needle grouping (in the case of tattooing), the smallest brush, or the sharpest pencil still has some width to it.
If you've ever had the privilege of seeing famous oil paintings in person (like the ones in the Met Museum up in LA), you'll notice many of them are quite large. They cover entire walls, and many easily tower over most people there. A large canvas is a key factor in allowing artists to include a high level of detail. While there are some impressive smaller paintings and drawings, you'll notice that a lot of the details then are more suggested than they are drawn in in their fullness.
I discuss this more thoroughly under the fineline and microrealism page with more examples, but it is understood that tattoos very slowly spread over the years as they age. This is separate from the phenomenon of blowouts, which happen fresh as a result of going too deep. Because the macrophages (white blood cells) that hold the ink have a lifespan of a few months to a couple of years, when those cells die, the ink is free floating until a new macrophage holds it in place again. This means that over the decades, it can look like the design is "blurred". So it is very important to allow for details to have "breathing room" so that key elements of the legibility of the piece are not lost too soon.
Different elements will have different levels of detail that are key to their legibility. And with some subject matter, it can be okay to lose some detail and still have a good read. For example, a piece of architecture that is complex like a gothic cathedral might need to be larger, as there are a lot of lines close together (example A versus example B on the right). But for a portrait, maybe it's okay if we lose some detail in terms of the individual hairs, as long as there is enough space for the facial features (like the whites of the eyes to not be "eaten" up by the ink surrounding it).
Aging and what looks good to each person is not a black and white limit. It is more of a gradual spectrum. This is why I always recommend that people spend time on websites where people share their aged tattoos, like this blog, to get an idea for themselves of what feels successful to them. That being said, I do have some guidelines for what I tend to recommend from my experience, and I will be sharing the reason why below!
Florals are a common subject matter that I tackle. I tend to go a bit case by case as each flower is so different, both in terms of their size in real life, and in terms of what details make them so iconic to us. That being said, for complex blooms like roses, peonies, and crysanthemums (where there are many layers, and each petal has its own shadows, midtones and highlights), I tend to recommend a minimum of about 3" of height for color or painterly realism.
That being said, it's important to also keep in mind that the bigger we go, the better the detail level can be. As we can see in the example above of different tattoos I've done of roses on the left, a rose done at about 8" (without accounting for leaves) can contain significantly more detail than a rose done at about 1.2" of height. These smaller sizes can work well for smaller versions of the blooms, like rose buds, especially if they are part of a larger project. But as you can see in the rework project where we were limited in size by the original piece (the example that is about 0.5"-0.75" tall), we aren't really able to fully take advantage of the complexity of realism.
With the examples on the right of the image above, I have also shown some examples of smaller flowers that I don't think would need to be so large. Namely, the rose buds, the peony bud, the poppies, and the forget-me-nots. These are either smaller and simpler versions of larger blooms, or are smaller flowers, that might not require as much space to be executed well. These typically work well as filler, or as a secondary or tertiary element to a composition.
Above are some examples of different sizes of facial features that I have tattooed. While it isn't impossible to do a very small face fresh, our most extreme end of the spectrum on the left does not lend itself well to the aging process (clients are always warned and given teh opportunity to give informed consent after this discussion). As you can see, at that size, the facial features are more suggested than truly painted in - the nostril and nostril shadow are no more than a couple of strokes at most, and the planes of the face have to be simplified.
At our 1" mark, it's still fairly stylized. The planes of the face are also simplified. This makes sense for this Klimt piece as it is a stylized painting, but I would not recommend it for painterly realism. On the 2.5" example, even though it's a partial face, we can include a lot more subtlety - things like more than one tone for the lips, defining the shape of the shadow between the chin and lower lip, giving the nostril a shape and the bridge a distinct form, even shaping the philtrum a bit.
For faces, I tend to personally recommend at the very least 2" of height, 3.5"-4" if we want a distinct expression and/or some level of resemblance. And similarly to our discussion with florals, the bigger we go, the more detail we can include. Going for this absolute minimum recommendation would not give us the same result as letting a portrait be the largest shape in a quarter sleeve, for example, and we are more limited in terms of not being able to include microexpressions at that size or elements like aging markers, freckles, eyeball or eyelash details, etc.
Wings are a fairly complex subject matter. They have primary, secondary, and tertiary feather groupings. Depending on the bird it's inspired by, they could have unique color patterns in each of these areas.
For color realism wings, I would recommend a minimum of 6" for each wing's fully extended span unless stylized and simplified somehow.
You can always fill in a form and discuss your goals in terms of level of detail for the piece, and I am always happy to share my best recommendation! If you do want to break away from artist recommendations, that's totally fine too. It's all about creating informed consent to the best of my ability. :)