When adding images to an iOS project, what size should the 1x image be?

For example:

If I have 2 `UIImageView` with the same image with a size of 50x50pt and 30x30pt. Can I make one image asset for the both (and which size should this be) or should I make 2 assets ie. image30x30.png and image50x50.png?

This of course extends to images that have a dynamic size (for example 50% of the screen width), which size should they be?