what variety of seedling rootstock is the best for grafting?
a kind that is less suseptable for diseases.

which grows faster an orange seed or a lemon seed?

Actually none is best. It all depends on where the tree is going to be planted, in what type of soil ( wet, dry, sand, clay), the climate of the area, and temperature. It also depends on if you desire a small, medium or large size tree. There a lot of orange varieties, and a lot of lemon varieties. Some grow a little faster and some a little slower. If your growing a seedling just for rootstock, the speed of growth really does not make much difference. - Millet

can you manage or constrain the desired size of the tree?

Yes, either by growing in a container, or by using Flying Dragon as the root stock. - Millet