The first tip you need to know is to measure your space. I know, this takes digging out a tape measure from the garage or that one long measuring tape in an old sewing kit and risking life and limb on a ladder. It is worth it. We measure our wooden monograms by the height of the center initial. In a 3 letter monogram, the width with be about the same as the height, but it depends on your specific monogram.
People ask me often, “What size should I get for above my baby’s crib/ living room/ dorm room?”. The answer is, I’m not sure- it all depends on your space! Do you have 12 foot ceilings? Do you have other pictures you’re hanging in that space? The monogram pieces will look the exact same in any size- just larger proportionately the taller they are.