Well if you do not have a scale you can eye ball the girth of the prey versus the girth of the snake at its most girthiest part.
You want the prey to aprox the girth of the snake at its widest part, and it is ok for the prey to be slightly bigger......slightly. I think from what I read is it is better to feed one appropriatly sized meal when you can than two or more undersized meals.
My new B/P was being fed one large mouse when I bought her. My first feeding the mice I got for her were noticably smaller than her widest girth. I fed her two and she took them greedily. She also was irritable until the next week when I fed her a small rat which matched her girth perfectly. Two days later she was more in line with the docility I had thought I should expect and was an perfect angel. Until she went into shed, but then that is expected from an uncomfortable and blind animal.