I quite like the penis_size_difference tag, as well as size_difference, big_dom_small_sub, small_sub_big_dom and smaller_penetrated_larger penetrated etc.
I like it when there's a clear difference between the characters fucking, and I also like when expectations are subverted. Most art will naturally assume the person with the big dick is always the top, or that the person with the smaller dick should always be the sub. So I quite enjoy when you come across those exceptions to this, when the person with the big dick actually prefers to be fucked, or when the person with the smaller dick isn't automatically a sub.
However this is really hard to search for right now.
Size_difference only tells you that the partners have different body sizes, the penis_size_difference doesn't tell you if it's the bottom or the top that has a big dick, likewise larger_penetrated/smaller_penetrated/larger_penetrating/smaller_penetrating doesn't differentiate if it's body size or penis size and many people use these interchangeably.
If we added tags for big_dick_sub, small_dick_dom, big_dick_dom, small_dick_sub then this would just make it much easier to search specifically for what you want. Some people want the sub to have a huge dick, some want the dom to have a huge dick, some want both to have huge dicks, some people might even want both to have small dicks. With these tags you would be able to find those images, without having to filter a large amount of images that aren't really what you're looking for.