On the Outwell website they give pack sizes - also depends on your reasons for wanting it small - sims can be quite heavy so not ideal for backpacking.
I cannot fault my Therm-a-rest Luxury Map XL SIMs as I was able to put them back into the storage bags they came in, providing I did the following (not sure if the OP needs this or not, however, just in case this helps):
h
Roll it up under pressure by squeezing it as tightly as possible with my hands, with the valves both opened.
Close both vials.
Unroll the mat.
Roll it up under pressure again.
Towards the end of the rolling process, kneel on the mat and open one of the valves to expel as much air out as possible.
Close the value.
Also have a look at the offerings at Alpkit as they have good reviews.
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