hi have been trying to work out why it is that when i create a split head / body morph from one i have sculpted this happens? can anyone please help?75% of original size (was 669x19) - Click to enlarge

This usually happened. I do not know how to prevent that, but the fix is to tweak corresponding bones for eyes and teeth.
Eyes is easy, teeth might give a little headache though.

From 3:32 in the video: https://youtu.be/HLSXq1NOxmk?t=212