What size of lens filter (UV etc) works on Canon 70-300mm f/4-5.6 EF USM IS Lens?

Answer 1

The canon L series lenses use the 77mm, the less expensive lens (that comes with the XTi kit for example) uses 58mm
As J man says - it should be printed on the front of your lens.

Answer 2

Look at the front of the lens, it says right on it.

58mm, just in case...

EDIT: Interesting... could it be a typo ? Perhaps a different model lens, 'cause I just double checked and I've got 58mm in the last two catalogs...Dunno

Answer 3

77mm