Painkiller nailed it...
But, I must add to his response...
I suggest that you do not make it larger than 1024 square...
But that depends on the file sizes of all the other rendered objects...
The reason I say this is because all the rendered objects can drastically affect your frame rate.
On that note, I also suggest that when you are happy with your texture file... convert it to a .dds file.
That type of file runs the best, as I've found, because the file size is much smaller than say a, .bmp or a .pcx or a .tga...
I also suggest that your animation is bones based rather than vertice based. Once again, bones is a much smaller file size...
Hope that helps...