There is a lot of romance behind the term "survival knife" much like there is behind "bushcraft knife". All the romance and BS aside these two knives are very similar, but I digress. To me an ideal size is something that I am likely to have with me on dayhikes, canoeing trips, backpacking trips, exploring, etc. This is because if I was going to get lost in the wood for days those are the activities I would probably be doing at the time. During those activities I carry a medium size knife, currently one with a 5" blade (I was carrying an Esee-4 but I upgraded to the longer, thicker, lighter Fallkniven S1). I would generally consider a hard use but portable survival/outdoor knife to have a full tang, comfortable handles, and a blade from 4.5-6.5 inches (3/16-1/5" thick) with a sheath that is comfortable and has good retention. That being said I sometimes swing to carrying a longer knife like a 711, depending on how I am feeling at the time. For me at least during day hikes I dont like the weird looks people throw at me when they see I have a huge knife. I see people who are lead to believe that they need a huge knife! I understand it minimizes work and all that but the likelihood that you are going to be lost during a day hike and need to baton through three cords of woods are is very slim. I think that the size of your knife should relate to the likelihood that you will need the huge knife. So that means day hikes a small-medium (4-6") knife is good and maybe for backpacking (miles out in the middle of nowhere not many people to be heard of) a larger knife is preferable (5-7"). That's my philosophy and thinking, I am still young and that is likely to change but I think a good many people share IMO.
Good Survival/Bushcraft ("Bushcraft" referring to the function not the romanticized/elitist BS) Knives to consider:
Fallkniven F1, S1, A1
Full Sized Kabar, MK1
Esee 4, LS, 6 (I wouldn't consider the 5 if you want something that can actually cut)
Buck Punk
SYKCO 511, 711
Swamp Rat RMD, Rodent 4, 6
Becker Tweeners, BK-7, New BK-10, MK-2
Bark River Aurora, Bravo 1, Bravo 1.5
Cold Steel Recon Scout
Ontario RD-6
Sog Seal Pup Elite
Sog Tech Bowie
Sog Force (Plain Edge)
Blind Horse offerings
Survive Knives offerings
Koster MUCK (That thing is so sexy, probably my dread knife) and other Koster offerings
I do not have or have handled (about half) all of these knives but the reputation of their respective companies and numerous online reviews lead to believe they would serve you well. I would not hesitate to use and abuse any knives on said list.
If you want something that will never fail you I would get an Esee RC-6 or A1. If you want something a bit smaller go for the RC-4, S1 (have one, LOVE it), or F1.
Toughness and cutting ability are not mutually exclusive. A thicker knife ground right can cut as well as a 3/16" blade. The Fallkniven S1 and A1 are both thick but because of their grinds cut very well.