the smallest wall hanging i've made is 15 inches and the largest i've made is 7 feet by 7 feet.
the hanging method is always based on the style of the design.
if i want the hanging rod to show i use tabs. if i don't want the hanging rod to show i'll use a sleeve.
when the wall hanging is an odd shape (circle, oval, etc) i sew small tabs to the back and run wire thru them to help hold the shape.
when the wall hanging is geometric with cut outs i'll typically build a frame out of thin wood and i'll use velcro to hold everything down.