TRY BEFORE YOU BUY
Seriously, go to a store that carries them. Call first to be sure they have the bags in stock that you want to try.
You want to check the fit - especially the shoulder girth. WM basically makes each TYPE of bag in one of two shoulder girths; regular (59 inches) and wide (62+ inches). Same bag; they just change the names.
Summerlite: 59 inches, 32°
Megalite: 64 inches, 30°, wide adds draft collar
Ultralite: 59", 20°
Alpinlite: 64", 20°
Match the shoulder girth to your sleeping style. For example, if I were to only sleep on my back, 59" will work. But I don't; I'm a side sleeper who rolls a lot. I need the extra space so I'm not compressing the down above my top shoulder when I'm on my side, and so my elbows have room to move.
If you're going to augment your bag with clothing, like an insulated jacket, in an effort to stretch the temperature rating below the default rating, take that jacket with you to the store as well.
WM bags are expensive, but the quality is out of this world. My Hooded Aspen MF - now the Sycamore MF - is still going strong after 15+ years on the trail.
BUT THE BAG HAS TO FIT RIGHT FIRST.