There are plenty of illustrations of splices with dimension, but it does depend on the style of bow, depth of grip etc.
My ones on the tips are 2.5" long, but being at the tip there is little leverage so they can get away with being only about 1/2" deep.
On warbows they need to be much longer than on an ELB. On and American longbow there is extra grip thickness that helps.
Best to go long rather than risk having insufficient glue line.
It's all down to educated guess work, also down to how good your splice is.
Del