Hey Twijfelaar,
Small hands is the key point of your post, IMO. The FP3 seems like it will stay nice in the hand (given the back, given reviews I read, and given the way it looks) but it will be a tad bit longer than FP2 (I read here on forum 10% but I did not verify).
With regard to hand size, the bigger smartphones are horrible for you, which is the recent trend, but as long as the size goes in length it is OK I suppose. What I don’t know for sure, is how the fingerprint reader is going to be for different hand sizes.
A second hand smartphone does not market itself as being sustainable. Other people don’t know or see this. But that is a bad argument to buy a FP3, one Fairphone would disagree with. So for you, perhaps an other second hand smartphone which suits your hands well is perhaps a better solution.
You might want to read some of the reviews. I’m aware of 2 in Dutch but you might appreciate them in other languages as well.