I have one stationery design wall that is 8 feet x 8 feet, so usually large enough for many blocks - one and a partial project.
But I also keep a piece of flannel about 42" x 60" that I use on a leftover piece of house insulation foam core board which can be tucked under a bed or behind an armoire when not in use. I have also used a flannel-backed table cloth as a design wall.
Jan in VA