Do you know how to 'repeat' the texture on your prim?
You don't NEED 'bigger' textures to cover a 64x64m prim.
You can, in the prim's settings, under TEXTURE... set the texture to REPEAT.
So if your texture's some paltry 128x128 pixel wonder...
You can set it up to REPEAT across the entire texture as many times as you need, to get that texture looking right.
The smaller you go with textures, the better off you are. I never go higher than 512x512... and frankly, I prefer smaller sized textures, to reduce rendering lag issues.
I use megaprims very often, and believe me... setting your repeats properly and using smaller texture files REALLY helps reduce region rendering lag.