everybody is different. However at 6'1" a 56 cm frame might not be that far off. I'm 6'1". I ride a Giant sized M/L, (which has a 57.5cm TT)
My TT bike is a 56cm. On the other hand my Merlin is a 59cm.
You just have to try it out and see if it works for you.
That said, abike that doesn't fit, is not a bargain.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.