Cropping is the reduction of an image to give a new document size. Though Adobe has change the meaning a bit since it is possible to add canvas via the crop tool.
Use image>image size and verify the print size is 20x24 and the ppi is high enough at that size that it will look OK. That does not necessarily mean it is OK to Up the values, as Photoshop has no way of knowing what the missing pixels should be, but it will do it best to guess. If the values are increased too much, it will not look good.
It is always better to start with a image that have enough pixels to start with, then there is no issue. But we are not always that lucky and can only try to make it work to the best of our abilities.
In other words, if it works, it works, if not, then oh well try again some other time with a better picture.