For international mail, the classes dont matter, so think of them as a monetary value. Depending on which size you have, second class are worth 30p or 47p (the 47p ones say "large" on them). Again, for international mail, you can use either size as long as the monetary value is enough to send the letter (the size and class is only used for internal mail).

Depending on the weight of the letter, the prices are as follows:

Under 20g: 90p
Under 40g: £1.35
Under 60g: £1.82

http://www.royalmail.com/portal/rm/content1?catId=400036&mediaId=53800712

How many 2nd class stamps would I need on a standard letter envelope going from the UK to the US? I'm not really familiar with these kinds of stamps. :oops:

Thanks!