4 inch scions for cleft / veneer are great for practical reasons -- the size is perfect for handling and not so big as to pose a risk of bending nor waste grafting tape / wrap.
But, in terms of take percentage, there shouldn't be much variance due to size -- a skilled grafter can get upwards of 90% take for anything from a tiny shield bud all the way up to a 6 inch side veneer (when grafting mango).
If you were grafting in the stone ages and didn't have access to plastic / fungicides / tool cleansing agents, then I suppose there could be a difference due to scion size (ie, a beefy foot long scion hanging out in the open might retain more moisture than a little scion needle). In that case your rate of take would be in the single digits.