When working on my son's old Lannutti MTB this morning, I noticed that his rear axle is slightly bent. Since I am likely to need to buy a replacement for this on the internet, I need some advice on size.

I know that the axle is 175mm long and that they seem to come in 2 diameters - 9.5mm and 10mm. I cant measure the diameter precisely and since the difference is so small, it is unlikely that I can measure the thing accurately at all! So...... is there a way to establish what diameter I need - measuring dropouts, etc? Or is it safe to use the smaller size?