You can either try before you buy, or match your existing glasses measurements.
When choosing new safety glasses you have a couple of options.
You can choose a pair that you have had previously that fit well, or even make those glasses dimensions. To know the size of your current glasses (they don’t have to be safety, they can be your normal glasses), read the markings on the inside of the arm/temple the bit that goes over your ear. The numbers are something like this 52-19 (this tells you the lens diameter and nasal bridge width.) So you can match your current measurements to those of the frames available.
Alternatively you can choose to have a small selection of frames sent out to you. This feature does incur a charge for administration and a deposit towards the frames – just in case they aren’t returned. Once the frames are returned and an order created this deposit can be moved as a credit towards this order.