Bike sizing.
 

If I'm 5'8" with a 30" inseam and have a recommended bike size of 55cm, would a 58cm (I'm getting a sweet deal) frame be to big?

Yes. Much too big.

I think so. I'm an inch taller than you with the same inseam and ride a 54. I could maybe push it to a 55 but nothing larger than that without a ridiculously short stem and buried seat post.

I'm 5' 9" with a 32" inseam and have a 19.5" (49.53cm) 3500 Disk MTB and it fits perfect. It has a 110mm/25deg stem with 3.5" of seat post out. Don't know what style you are looking at but that's my .02 worth.

Yes too big. I'm 3.5" taller than you and most if not all contemporary size 58s would be a size too big for me still.

I am 5'7" with 29.5in inseam. I ride a 52cm. I would say a 52-54cm would be a nice fit.

too big. i am 5 10 with long torso short legs and can do 54 to 56.

I am 5'9" and ride road bikes from 53 to 57cm. I do have to put short stems on the larger frames but have no trouble riding them my two favorite rides are 57s. I buy used so can't be size picky.

Would you wear a pair of shoes if they where 3 sizes too big? If it's the wrong size it's not a good deal for you.

It's not a sweet deal if it doesn't fit. Nobody can really say without seeing all of your measurements and some bikes are sized differently than others, but it's highly unlikely that you'd fit on a 58. A "normal" 5'8" guy wouldn't, at least.

I say that as a 5'7" guy who rides ~54cm top tube.

The other issue related to a longer than optimal (for you) seat tube is the corresponding longer top tube on standard bike geometry. (Custom made is a different story.) You'll not only be stretched to the legs, but in the cockpit, too, in all likelihood. You can play around with steam length and handle bars, but you run into potentially destroying the handling characteristics and the deal becomes less sweet as you sink money into the compromises. At the end of the day, long term happiness is unlikely.

3cm in top tube length is a big deal. i just shortened my reach by changing to a stem that was 2cm shorter. it is very noticeable the change in reach and how much more fatigued i felt when i was 2 cm too stretched.

Frankly, a 55 is bigger than most people your size ride. Unless you are very unusually proportioned, you might want to seek a 2nd opinion on your ideal frame size. 52-54 would be where I would be looking were I your size.

52-54, I ride something similar to that and i'm 5-11 and at times i feel it's too big.

AT 5'7 1/2" with slightly longer torso and shorter legs I have always ridden a 54 cm c-c seat tube and 54.8 cm to 55.5 cm top tube. I am still able to use a 110 mm stem. Anything bigger than 54 cm for you is likely too big.

Buy it, nothing's better than getting a sweet deal on a new garage ornament that is 3 sizes too big for you.