These R1s look remarkably similar to the FCS Stretch SF4s...maybe halfway between the SF4s and R2s
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The SF4s are supposedly M but they used to be categorized as L, but sometime recently FCS changed their mind. In any event, I'm 90kg and I find they go OK on my dom.

Here's the complete measurements for them:
Future Fins Rusty R1 5-Fin (Tri) Fin Size:
Outline: Rusty
Height: 4.54 in. 11.53 cm.
Base: 4.45 in. 11.30 cm.
Area: 16.75 in² 95.16 cm.²

Future Fins Rusty R1 5-Fin (Rear) Fin Size:
Outline: Rusty
Height: 3.86 in. 9.80 cm.
Base: 3.55 in. 9.02 cm.
Area: 9.93 in.² 64.05 cm.²

The front R1 area is slightly less than that of the front SF4, but area alone isn't the whole picture and while R1 is 115 (h) x 113mm (base) compared with the SF4 114 x 114mm, I can't get any info on the rake of the R1.

More rake can stiffen a fin and make it more suitable for a heavier surfer who can flex it and get it to go like a looser fin anyway. So as near as I can tell looking at the photos the rake looks similar to the SF4 maybe a little more. Those rears are quite small too, so I think you may be right. Big difference between them and the Futures AM2 in size. It looks more like a medium...but some medium fins work for me eg. SF4 & K2.1

Yeah Slowman, good to hear you're getting on ok with that kind of fin. I've had a few recommendations on the rusty set (from bigger guys too), but I'm still not sure.

Was thinking about getting a set of SA4's instead, but it doesn't look like any one is doing them in the UK. Was also thinking maybe the rusty blackstix tri with something else for quad rears.

im 215 6'4 and ride a spitfire 6'6 with these rustys as a quad and love them.i also have switched out the front fins with am2 tech flex's when surf is a little bigger then chest high for more hold and drive.but yet i still find myself going back to all 4 rusty quad fins.where i surf waves are more waist to chest high and the rustys stay in my board more then any other fins i have.hope this helps

yeah i agree with vow. from everything i have heard, the R1s are super versatile and get along great in both the quad and thruster.

I emailed futures to ask them what they think, and the guy reckoned I might be to big. Thing is, I've heard from a few bigger guys like you vowbeachbum who say they love them. I've got a set of AM2's too, so could be swapping them in as you suggest. It was the quad rears the future guy thought were small - he thought they'd go great as thruster. I'd almost decided on a set of SA4's instead, but now I'm undecided again..

Just buy both sets, then you can't go wrong!

have you tried the Josh Mulcoy 5-Fin as well? If so, how does it compare to the Rusty R1?

I have the R1 set in my Hellfire and I love them. Great hold and tons of maneuverability on the wave.

@hwork50, thanks for the feedback...ill try to demo those fins soon!