I got a large plastic clear floor mat (with spikes to keep it in place) and it lasted less than 4 months before slowly disintegrating.
What I presume happened was that with my chair wheels being in near enough the same location on the mat and the thick soft pile of the carpet the pressure created progressive dents in the mat - and I'm only 13.5st thereabouts. Those dents will then crack and once it cracks, that's it - the cracks will spider outwards, the wheels will catch on them and make it worse. I'll post a picture of the state of the mat if anyone's interested.
Here's what I'd do, and I saw it first on these forums but haven't searched for the exact post - is to buy some Click laminate flooring and make your own to your specific size. It'll last a lot longer than those crap mats.