Here are the options I would consider:
1 - email REM customer service or check their website
2 - remove sight and measure dovetail
3 - look at aftermarket sights and see what size they are listed as
4 - internet search engine
5 - compare to another pistol of known dovetail size
Let me help...from their website.
What sights will fit the 1911 R1 factory dovetail cut?
The 1911 R1 utilizes a standard Novak type front and rear sight cut. Other sight options available through outside sources will need to match this dovetail cut in order to be installed with as little potential for damage to the slide as possible.
Good luck and welcome to the forum.