well,
for starters, breeding can be very dangerous. But if your up to it, Both dragons should be at least two years old and at or close to 800 grams.
Length isnt an issue unless one dragon is very small and the other is big. You should do a winter time cool down and get them both tested for adeno virus. (This can be done at your vet) Just keep in mind that babys are VERY exspensive. they will probably go through 1000 crickets a week or more!
good luck with your breeding though!