I’m printing a 17X22 from Lightroom.
I crop my image 17X22 and when I go to the margins in Print the bottom is .58 in. All the rest are .25, which I want as the default all around. The bottom won’t budge even though I try to type in .25.
Generally printers have minimum margins, below which the margins cannot be reduced--unless (1) the printer has a borderless mode, (2) borderless mode is available for the size of paper on which you're printing, and (3) you turn on borderless printing. It may well be that 0.58 inch is the smallest bottom margin your printer offers for 17x22 inch paper. In Windows you have to select borderless mode in the printer driver. Maybe on a Mac you can select it in Lightroom itself--I don't know.
As Mark said, in the Lightroom Print module, if under Lightroom Templates you select Maximum Size, that will get the margins as small as they can be. If the printer is set to borderless, those will be 0, but otherwise they'll probably be as you reported.
Also remember that if you crop your image to 17x22 and then print with those borders--or even 0.25 inch borders all around--either a little of your image will get cropped out, or some extra white space will be added, because you've (slightly) changed the aspect ratio.