If you can straddle the bike without crunching your privates, you can probably get by OK. 56 - 61cm sounds like a perfectly reasonable range for someone 5'11". I am 6'0" (on a good day) and I ride 60 - 64cm bikes. I am probably more long legged than most. Sounds like the way you are measuring, the bike in question should be properly identified as a 61.
Once you get used to riding, the length of the top tube will become at least as important as the seat tube.
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● 1971 Grandis SL ● 1972 Lambert Grand Prix frankenbike ● 1972 Raleigh Super Course fixie ● 1973 Nishiki Semi-Pro ● 1979 Motobecane Grand Jubile ●1980 Apollo "Legnano" ● 1984 Peugeot Vagabond ● 1985 Shogun Prairie Breaker ● 1986 Merckx Super Corsa ● 1987 Schwinn Tempo ● 1988 Schwinn Voyageur ● 1989 Bottechia Team ADR replica ● 1990 Cannondale ST600 ● 1993 Technium RT600 ● 1996 Kona Lava Dome ●