Everyone here has brought up very good points. Dwarf retics is a relative term. They can still get up to 15 ft for a female. But whereas a normal is pretty much guaranteed to get over 15 ft, this helps. The big difference that I have noticed between normals and dwarfs, as far as stats go, is their wieght. A 12 foot snake that might weigh 50 lbs, is a big difference to an 18 foot snake that might weigh 150 lbs. This is pretty much going from a snake that one person can handle relatively comfortably, to a snake that would take 3 people around to really be safe.
I love the big snakes, and I actually have heard of people crossing dwarves and normals. Not for morphs or anything, and as far as why, dont ask me. The dwarves that have been bred with large sized, to gain morph genetics, will be bigger than a regular dwarf. There are some dwarf albinos out I believe, or at least hets. If you have to stay under 15 feet, be sure to get a male, and dont be surprised if it gets a touch over that mark. Just keep sliding the string forward when your girlfriend wants you to measure it.
If you get a male dwarf, that is pure dwarf, you really shouldnt have a problem staying under the 15 foot mark. Even most female dwarves stay under that mark.
Good luck
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1.0 Albino Green Burm
0.2 Normal Burm
1.1 Java Retic