Depends on the type and how much of a medication you’re injecting and how you administer it (SubQ/IM). Needle gauge size also can impact comfort and bleeding as well. Depending on where, needle length is also important as some areas have more body fat as you’re aware, so may need a longer needle to accommodate.
Thinner needles have higher gauge numbers while the thicc bois have smaller gauge numbers. When I was still injecting my oestrogen, I used a 28Gx5/8 needle size for my injections and would use larger ones (22G-20G) for drawing from the vial. Testosterone would be better administered with a larger needle (smaller gauge number) because it’s thicker than oestrogen. The pharmacist should be able to answer any of your administration questions, too!