I am also a new quilter and I started off with a Fiskers' beginning quiting set I got from Joann's Fabrics.
It has an 18 x 24" cutting mat, a 6 1/2 x 24" ruler, and a 45mm rotary cutter.
If you buy the set when they have the 40-50% off coupons, you will only pay between $25 and $30.00 for the whole set.
I have never had any reason to use a larger mat, but have only been quilting for a few months. For the price it is a good start.
I just checked online, the regular price is $47.99.