I would try to find actual measurements of what you're buying.
Single and twin have always been synonyms for me. However, a single bed in the UK is differently sized than a twin/single in the US. An extra-long twin in the US is differently sized than it's UKian counterpart.
Not so sure about the larger sizes though.
Here's some tables of sizes. I don't know how reliable they are.
http://interiordec.about.com/cs/beds/l/blbedsizes.htm