This is a nice size foam mat, big enough for most projects. This is the size we recommend for our 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.
Measures 10 in. square, and 2 in. thick.
Last picture shows this mat in comparison to our small mat that is 5 in. square.