While the OED might be accurate, it is certainly not the standard English resource for American English, except for maybe people in academia. I consulted Merriam Webster's and found life-size and life-sized. Merriam Webster's would be a more appropriate resource if you are in the U.S. or have an American English-speaking audience.
As for what to use outside of the U.S., I would trust the judgment of the other posters.
But, regardless of what you use, it sounds like you will technically be correct.