Im with Cav on this one, as regards small breeds (not sure about larger breeds), but i will hopefully be mating my Cavaliers next year subject to health test results etc, so to play safe for my females first time i would opt for a smaller male, or at least one of the same size to hopefully prevent any birthing problems, i wouldnt be so worried if she had already had a litter but for a maiden i would deff go for a smaller male.

This is just my opinion and the way I will be doing things, I'm sure there will be more experienced breeders that can help too.