If you have no tools at all ...

A Power Plug and a Needle
from the travelers kit that you normally find in hotel rooms were my most frugal sizing tools ever. I had to resize a Seiko 5 Sports re-edition for a friend and I did not bring any watch tools with me at all. Thus I really had to improvise. I found a needle in the traveler's kit in the hotel bathroom and used the power plug of the tea kettle to hammer carefully down on the bracelet. And quite surpringly, I could resize the bracelet without doing any damages to it...

PS As a polite hotel guest you might want to use the phone book as a working surface, it also stabilizes the needle fairly well.