I like to use things that can be found all over the world. Tubes of lip chap, a Venus Razor and a tube of mascara. If I had any type of carbonated beverage in the house I'd probably use that as well as that is a standard size. Originally I was going to use a passport as those are all the same sizes, but for many months of the year it is not at home!!
I find things like pens just don't work as everyone in the world has different sizes.
I love seeing comparisons with other toys of the same type so I always include those.