Heparin/Lovenox syringe gauge question

Jennifer
Good morning!
I'm a week into twice daily heparin injections and am almost out of sample syringes that my doctor's office provided. They gave me several sample sizes and I'd like to try a smaller gauge if it'll work. What sizes do you all use? I've been using a 25G. Has anyone had success with a 31G? 
Thanks!!