This is a nice size foam mat, big enough for most projects. This is the size recommended for thr painting with wool kits and coloring with wool kits which get framed in a 7 in. embroidery hoop.
Most people who are new to felting choose to start on foam to keep costs down while they are just trying it out. Keep in mind a firm foam mat will hold up awhile (depending on the types of projects you're doing) but will need to be replaced once it starts to break down.