Originally Posted by bigredgrad01

I am 6 foot, 230 lbs, and I went with a 56.
It's somewhat unusual for a size 58 bike to come with a 90mm stem. Did your bike come with it, or were you too stretched on what it came with and ended up getting a shorter stem? That would be a strong hint to go to a 56.
If you aren't sure between 56 and 58, I'd go with a 56, so you can always just get a 100mm or 110mm stem to dial in the fit and have slightly more stable handling. But if you get a 58, you will probably have to swap the 110mm stem for a 90mm, but then if you are still too stretched, then going to an even shorter stem will result in more twitchy handling.
Also, keep in mind that 56 comes with 172.5mm cranks and 44mm wide bar, while 58 comes with 175mm cranks and 46mm bar. I prefer 172.5 for less chance of pedal strike and easier spinning, and even 44mm bar was way too wide, so I swapped it to a 42mm (40mm at the hoods) 3T Ergonova, but you might have different body type and preferences. I would recommend test riding both if you have a chance.