There's a lot of good face makers doing master pieces and spending time to increase the potential of the game. But I'm having some troubles in installing new faces. When I do substitutions, in a team that use imported faces which use different texture size of Fifa 08, some player appears with no texture at all on their faces (absolutely white). Fifa 08 use 512x256 for the face texture and 128x128 for the hair, but some face makers use different texture size for their faces.
So, my question is: Is there any workaround for this problem? If I resize the
texture (face and hair texture), the face still work?

Yes man, you have to resize the face always to the sizes you have just said:
face: 512*256
hair: 128*128
Other sizes (less or great) give problems ;)

And how can you see the size of the O file or the fsh files? And how can you change it?=

well I think less sizes are possible, too. In german community many people imported BOF3 into FIFA08 (all faces are 1GF) and it works fine.