although when I went back they had stopped stocking plain mats and only had ones with phrases such as 'Welcome' on them...
Theirs are not cheap, but you can get them from IKEA.
True story. Last summer I visited about 15 different stores (supermarket, furniture, hardware, garden, junk, you name it) with no success. Every single mat had some garbage printed all over it with no other purpose than making it:
1) a "design item", doubling the price and
2) unusable for gaming.
My previous rant may be here somewhere already. Fortunately I found other useful and/or interesting stuff on that tour so it wasn't three days entirely wasted.
Finally, a friend dropped me a hint on IKEA mats so I grabbed one. It was maybe 10e, I think, so not exactly a bargain but will cover all my agricultural needs for a whole table. I felt filthy after visiting that place, though.
Coincidentally, only last week I found enough time to start chopping and fine-tuning it. There's some progress already.
Regarding the original topic, I agree that bigger is better when it comes to realism. The reasonable size in 28mm or even 15mm would be larger than a standard table or you'd be turning a combine harvester around every minute. However, for my skirmish project I use multiples of 6" (15cm) so the tiles are 6x6, 6x12 or 12x12. There won't be too many border lines, while the individual tiles are still portable. Furthermore, my Renedra fences and some others are about 6" so I can enclose the fields neatly.
Finally, try to add some colour and details to the mats if only possible. Just plonking a piece with its grey rubber edges and all is a bit too cheap to me. Coir mats are a fine starting point but not quite there yet. It's fairly easy to go one step further with customisation.