I don't have rubber matting but if I did I would lay a full bed down, or one isn't quite as thick but still a decent size. I hate seeing stables with just a sprinkle of bedding down because they have rubber matting.
I just put down a small square of shavings to one side. Ive found all mine happily lie on the mats but do like the shavings square to wee and poo on (stops pee splashing!). They all seem pretty happy with it.
But then again I only have an old shetland who comes in every night! So she pretty much can fit onto the square to sleep. With the other horses, they somtimes come in for an event the next day in which case they have the same. During the day I like to sweep them all to the side so the mats dry out during the day.
I have tried allsorts with my messy mare including minimal shavings, big shavings bed and I now have big straw banks with a thin layer over the floor and it works really well and has saved me a fortune this winter, she wee's down one bank s I just remove that every other day and the straw on the floor stops all the poo sticking to the matting, the only thing I don't like about it is it can be quite smelly!
I lay a half to two thirds bed of shavings of a normal depth (6 to 8 inches), and then my horse rearranges it all over the stable to suit himself. He has plenty to lay on, but it is cheaper than giving him a full bed.
We have a spinkling to stop the splashing, probs about a bucket and a half/2 buckets'ish. The horses are happy to lie down and do everything they would if they had a full bed but it makes it a hell of alot cheaper and easier. The matts are pretty soft. We've never had any horses cast or capped hocks and all our horses are shire or percherons and crosses there off.
Depends on the mats and the horse's lifestyle! Really thin mats are only suitable to be used under a fullish bed. Top quality springy mats are fine with just a pee-patch if the horse is only in at nights. For horses that are in for longer then more bedding is usually necessary.
I have rubber mats but still have big banks and a decent size bed but it doesn't cover the whole stable (12x14). Used to have bed size of stable before mats so I don't use as many shavings per week.