Checked a few websites:
tempurpedic.com 53x74 three different models
samsclub.com serta 53x74.5 two different models
sealy.com listed 53 x 75 as standard , with a disclaimer that various models vary by plus or minus 1/2 to 1 inch.
Seems like there's a lot of variation in "standard" sizes out there.
The inner shell of the Oliver is fairly squared off, just a little rounded curve in the two corners. A full size good quality pillowtop mattress pad fits snugly on our Oliver cushions, and for us is very comfortable. The foam they used in our cushions is very dense, provides good support, and we do not "bottom out" sitting or sleeping. You may want to try them out first before investing in an additional mattress.