I'm about 5'11 in height, what is the best fixie frame size for me?

The size that fits you.

Lol do you wear Visvims or something?

I would say a 56 or 58 depending on your body proportions.

I'm 5'11" and ride a 52cm. Squirrelli rides a 58(?).

There's no way to even guess without getting specific body measurements, a list of bikes the guy is interested in, and a photo of him on a bike.

I'm actually only 5'9"ish but I just have long legs.

And yes, I ride an IRO 59cm that is too big for me top tube wise but perfect seat tube wise.

Recumbent.

1. measure inseam
2. report back
3. or refer to manufactuer spec item "standover-height" for roughest estimate of size
4. begin by choosing size with standover-height right below your inseam

*5. you must be able to do math: for converting millimeter (mm) to inch (in,") if neccesary
6. get bike that maybe fits you. win.