You don’t really need to think about the size, because the video card driver will move the texture in and out of memory when you use glTex and glDeletetex.
But one thing you have to think about though is that if you working set is huge, maybe all the textures won’t fit into your video card memory, so then the driver will put them in main memory or in Virtual memory(depending on the size of your ram). This will slow things up if their is a lot of swapping.
But you shouldn’t care about it too much. Just test your working set and don’t worry about the size, and if in the end you that your program is not fast enough, well one the the bottleneck could be texture size, so you’ll have to load less texture in memory and then you’ll be able to put a MAX_SIZE for your program.