What size frame?

My better half is buying me a hybrid for xmas
just wondering what size frame people reckon I'd need...I'm 5ft 10. Obviously I'm going to go to an LBS and check some out but just wanted to have an idea to start with...any help much appreciated.
Cheers

Cheers
"I think the phrase rhymes with Clucking Bell"
FCN = 4
FCN = 4
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That said, if it's a Trek then 54cm.
www.bikefitting.com
I think some of the better shops have a jig setup which measures you, it may be free if you buy a bike but not sure as my local shop that had it shut down
I find that reach is more important than the height as the saddle height is easily adjustable - cutting an inch off the top-tube of the frame is a little more difficult to achieve!
Am going to have a test of a few anyway before deciding but prefer having an idea of size before I go. Thanks guys.
FCN = 4
it's a hard life if you don't weaken.