PS
Regards you size matters debacle, consider this.
I shot reasonably well with an old £100 rifle in an FTR club. It was some years before my wind reading ability warranted the outlay of a dedicated FTR rig (some might say it never warranted it).
I still have that rifle and is shoot very well out to 600, it will go futher but I would have to hammer the old girl and I like just shooting her with a sling at 600. I no longer have the dedicated FTR rig.
Consider whether you are going to shoot comps or not, and if so will the rifle cut it or more importantly be legal for your early attempts.
Also, consider how often you are likley to shoot past 600? If its once in a blue moon buy the 20" rifle and go enjoy. My AR with an 18" barrel and poxy 556 bullets does nicely enough mag rested at 600 so a 20" 308 should give you a load of fun.
BUY the best glass you can afford and try before you buy, glass tends to be kept, rifles move on.
IF you fancy shooting dedicated long range shooting here's the drill.
1. Develop a 1000yard stare....you will be doing it a lot.
2. Have you brains kicked out...I did, it helped
3. Learn to accept that s*** happens
4. Re motgage the house, you will need it for the latest piece of kit everyone needs because X uses it and he wins (oddly the bloke/lass might just be able to shoot regardless). We had an odd influx of March Scopes into one club I am in, the scores never improved, they just missed with more panache
5. Get a divorce
6. Set up home on Stickledown
In short go with your gut instinct. There are loads of rifles that are good at long range and people just love to pop up and tell you what they are, but many of them require dedicated long range precision chamberings which need heavy barrels, highly developed hand loads, and will burn out in a season. They also require experience to get the best out of, best learned on something cheaper (Christ, I sound like a right condescending t*@t, I just dont want you to do what I did and slip into a bottomless pit of spending in a discilpine I should never had started....I was slow to realise and in effect stopped shooting for fun)
A 20" .308 offers so much in terms of flexibility and one of the widest choice of ammo and components ever (as well as info to go with it).
I agree the 24" might help, but my 26" is no long range precision rifle so do you need it?
Crack on mate and enjoy