Ultra 7R

Usually I have slept on gravel and rocks for 40+ years, as I am a Real Man (c) (tm) (PatPen). Now I have a Ultra 7R Long Wide sleeping pad from Enwild, and gosh I wish I had had something like that for the past 40 years. For side-sleepers, it is helpful to have a sleeping bag that includes a way to lash it to a sleeping pad.

I've got an Exped 5R in a long/wide size. It's listed as 77.6" x 25.6" x 3". I had a 7R a couple of years ago in the same size. It was a 3.5" thick mattress though. Honestly they've been some of the comfiest sleeping pads I've owned. I'm pretty much the same size as you are as well. I'm 6'2" and 215. I've never found the size of the pad to be too short or narrow. And I toss and turn like I'm clothes in a dryer. There's plenty of thickness to keep you from sinking in even if you're a side sleeper. The only downside to the pad is it's packed size. Which is bulkier than the traditional suggested sleeping pads, but it's really not that bad for what you get. Also I just looked at the Megamat. The 5R has a higher r-value, is still 3" vs 4", but that megamat packed size isn't worth the extra space at all in my opinion. I think the bulk of the megamat will cause more problems than it solves in the long run.

Yeah 8 is pretty crazy, I have an exped 7R and sleep with it literally on top of snow with no problems. Slept in -20°C a few weeks ago and the pad was fine but my bag wasn’t warm enough

I love my Exped. I have the 5R and the 7R. I like the size of the 5R better.

Get something with a super high R rating - higher than you think you need. Look for a baffling system that is either dimpled or vertical. For whatever reason, horizontal baffles don’t have a lot of “give” to them so they are less comfortable for side sleepers in general.  Therm-a-Rest Neoair Xtherm is probably the warmest pad out there (not the highest R rating per se but the most effective insulation system means it is still warmer than other pads with higher R values). It may not the most comfortable for a side sleeper because of the horizontal baffles, however. But this is the pad you want if you want to prioritize warmth. If you want to prioritize comfort, Sea to Summit Ether Light XR Pro will probably not sleep as warm but will be more comfortable for a side sleeper because of the dimpled baffling system. Nemo Tensor Extreme Conditions is also very warm and only slightly less comfy than the Ether Light.  Exped Ultra 7R is a bit heavier and bulkier but is very comfy for a side sleeper. It uses down rather than reflective film for insulation. So it won’t be quite as crinkly as a lot of warm pads, but I’m not sure if the down insulation is more or less effective in this application.  Big Agnes Campmeister Deluxe is quite bulky and heavy for a backpacking pad but is super, super comfortable and has a very respectable R value of 7.  The other big, bulky “luxury” comfort backpacking pads - Exped Megamat Ultra and Therm-a-Rest Neoloft - are great options for comfort but with lower R values so they may be less warm than you might want.  Other things to consider - even a well-insulated sleeping pad will only stay as warm as your body is able to heat it. The bigger the pad, the more of your body’s heat it will draw away from you. So don’t buy a massively oversized sleeping pad thinking it will automatically be more comfortable. For insulating purposes, something that is only as big as you need it to be will be more efficient for your body to keep warm.  Make sure you take advantage of all the usual strategies for staying warm in the backcountry: Use your Nalgene as a hot water bottle and keep it in your sleeping bag/quilt with at night. Do some jumping jacks or other activity to get your heartrate and body temperature up before going to bed. Make sure you are properly hydrated and getting enough food to keep warm. Etc. 

You could see if you can find the Exped Ultra 7R or the Exped Dura 8R. I saw the ultra around for 180 € several times. The delamination issue is mostly caused by the fact that the Exped Synmat was/is used by the german military and a lot of people bought those used from surplus stores which means the mat could be quite old already. Exped gives 5 years warranty but obviously not on surplus stock.

Exped Synmat Ultra 3R. The comfiest pad for side sleeping that I have ever found, and it doesn't make the awful crinkly noise like some other mats (e.g. thermarest) do. Its pretty robust too. If you want a higher R value, they go upwards in model number - e.g Ultra 5 or 7, but the Ultra 3 is perfect for 3 seasons.