The short answer is that the better choice is size, not sized—king-size and mid-size—although some dictionaries list king-sized and mid-sized as alternatives.

Which is correct size or sized?

The traditional rule is that an adjective should be modified by an adverb, not another adjective, thus: variously-sized documents. "Size(d)" in these examples is not an adjective in the narrow sense in which adjectives require adverbs, and "variously", tho ... better analog, so I would make it "various-size documents".

What is the difference between size and sized?

As verbs the difference between sized and size

is that sized is (size) while size is to adjust the size of; to make a certain size or size can be to apply glue or other primer to a surface which is to be painted.