I have an 18 x 24. I also got the quilter's beginner kit almost 8 years ago. I still have the same 18 x 24 inch mat. Sometimes, when I'm squaring up whole stripped rows or cutting off the excess batting after it's quilted, it seems too small. For the most part, it fits all of my needs.
I think it's a good, midsize, mat to start with. I got mine with a 6 1/2 x 24 ruler and a 45mm cutter. I still use them all regularly.