What size gravel bike?

If I ride a large mountain bike would I also then ride a large road bike?

Probably, although does depend on how flexible you are.

You can compare frames using https://bikeinsights.com/

I use it to pick a frame size when I'm buying a new bike, just get the closest fit to an existing bike I like. Although comparing MTB vs road/gravel will be a bit tricky...

I would never think a 5 9 person would need a large.  Do Orange actually do proper sizes (52, 54, 56 etc,.) rather than S M L?

I'm on XL across the board.

Sunset MTB had some RX9's at a great discount so maybe worth having a look at?

At 5"9", I'd say you could get away with a 56cm if you liked a stretched out aero fit, otherwise I'd say 54cm all the way. Obviously, you should really try before you buy (and get a bike fit).

Mountain bike sizes over the years have strayed from the old 'measure your inside leg and buy a bike based on that'.

Some makes of mtb's frame sizes only really change in length, the actual frame height doesn't change as much.

It would appear you're smack in the middle of frame sizes according to Orange. I think you need to try one out.

Have you or do you ride a road bike? To be honest that's a better measure than a mountain bike. A gravel bike is essentially a relaxed road bike fit (slightly less reach, slightly higher stack). A lot of mountain bikers sit on a road bike like a sack of spuds and have never really learnt how to fit themselves to one. I'd try and swing your leg over a few.

I'm 6ft and usually ride a 56cm/large. I'd say you were 54/medium unless you are looking at whytes which have weird sizing. Best thing to do is go to a shop stocking the bike you're interested in and try one for size rather than ask internet "experts" though.

I’m 5’ 9", I normally ride a 54cm road.

however, my gravel bike is a cube, which uses shorter top tube, so it’s a 56cm.

so, yeah as above, you really need to try before you buy.

I am a fraction taller than you and almost always ride a medium mtb, though I can get away with a large if I have to. I used to ride a 56 road bike, but on gravel I find a 54 is better for off road handling and position. That’s put me on medium test gravel bikes in lots of instances, but sometimes on smalls. Never a large though.

I'd go smaller than a road frame. Head tubes on gravel bikes are usually taller than a road bike. My pinnacle arkose has a 17cm head tube. My road bike and MTB is around 10cm.

One size smaller for me would enable the use of a dropper post and better saddle bag clearance.

I didn't have much choice when I bought my bike. The price was right so i bought it.

I'm always large on MTB's, have been for decades and even now I've got a (reasonably) LLS HT, still large - felt a tiny bit stretched at first, but it isn't - it's spot on now I'm used to it. I'm 5'10.5" with long legs/short torso

I was convinced I'd need a large gravel bike based on this, but wherever I looked, was guided towards a medium Boardman Adv 9.0

I went with a medium and I'm glad I did - a large would have been too stretched out (a road bike may be a different scenario). I flipped the stem (upwards) before my first ride and whilst it obviously felt very different to my MTB's, it was fine for a 35 miler and I used all the bar positions

I’m 5 foot 9. I ride a Medium Orange Crush.

I vary on road/gravel bikes. When racing road I preferred a 52cm frame with a 110mm stem (Trek H1 racing fit, quite aggressive geometry). Now I’m older I’m on a 54cm bike. My gravel bike is a Planet X Tempest size small, I still have a decent sized drop between seat and bars and a long stem. I could get by on a medium (54cm) but nothing bigger than that.

Not sure if that helps?

Dunno.. but ignore all this 56cm 54cm stuff, it's almost irrelevant these days unless you're comparing to a trad geometry road race bike fit.
Look at the frame reach, top tube length and ST angle and remember you can go +/- 10cm on the stem at least, plus bar shape and position/angle can affect reach feel to a similar extent.

I’m 180cm (5’11) and ride a large/56cm MTB/R/G.  But there’s more room for adjustment in a MTB than there really is in a RB/Gravel bike.  I’d suggest getting to somewhere where you can try a 54 and a 56 and see which fits best.

Reach, TT, etc are different for positioning on road and MTB, so there’s less commonality of measurement.  You need to go sit on a few, especially if it’s your first.

+1 on "you need to try something" if you don't know your road-ish size or preference.

The other thing that will affect sizing is your body proportion; if you have long torso for your height, you might well be more comfortable on a longer top tube. (I am 183cm, my large Revolt is pretty much a 58, but works for my long upper body).

Thank you all, be shop that somebody indicated above is only about a 2 and 1/2 hour drive from me. I think I'll give them a call and then I'm off the week after next so I might do a long road trip to Wales! Hopefully they have all of the frames in stock so that I can try them

Give them a ring first and I'm sure they'll sort you out. I've had a few bikes from Sunset and they've always been super helpful.

I’m a 56 a road bike, 54 on gravel.

If I’m in doubt or in between then I’d much rather change stem length and saddle position on a smaller road bike to make it bigger, than changes to make a bigger bike smaller… even more so on gravel.