The biggest factory wheel is a 19" and I believe there's a member with 20" wheels.

I personally wouldn't go over a 17" or 18" wheel on a base model.

 

The biggest thing you have to worry about is weight.

 

18" on a base model could be problematic due to different control arms and the turn radius.

 

So could I put Focus ST rims on mine or the stock black rims with the silver outline?

 

Go to tirerack.com and americastire.com to see aftermarket options. Pay special attention to TireRack's explanation about "construction method" and specifications for wheel weight. America'sTire has a handy calculatorhttps://www.americastire.com/learn/tire-size-calculator to compare different tire sizes, and also shows wheel weights.

 

anything from 15 to 19" diameter, width from 6 to 9", bolt pattern 5x108, offset in the ~50mm range.

towards the extreme of the range, then you might need to be concerned about fitting, but most will be just fine.

all Focus 2012 to 2018 oem wheels will fit.

 

I recently installed 17x8 wheels with 42mm offset, and they fit just fine with the 245-40-17 tires.
FWIW, my car is lowered around .75"

You don't really explain what your objective is for new wheels... looks, performance, or both.
As mentioned, I would be very, very careful not to add more weight than the factory setup. Extra weight will negatively impact ride, handling, acceleration, and braking.
My new 17x8 are 16.8lbs each, so a drop in weight. I got them for $110 each new. It was a great upgrade.

 

You don't really explain what your objective is for new wheels... looks, performance, or both.

this let us know what you are looking for...

 

I installed 18x9 wheels with 35mm offset and 225 - 45 - 18 tires.

0 problems at all.

 

Happy with my 19” RS wheels!

 

It would be fun to swap them with something lighter weight like STR5Guy! has and see if you could tell the difference from the driver's seat.

 

I have 18x8 ST wheels on my 2014 SE sedan and I don't have any problems. I'm looking for some 18x9.5, offset+40 for my car. Does anyone know if that size will fit and what size tires to use. Just for looks.

 

there are quite of bit of Focus ST people who put aggressive wheel fitments.
focusst.org might have more for you to search.
https://www.focusst.org/forum/focus-st-wheels-tires/38906-18x9-5-35-a.html

 

I have 235/40r18 tires on it now.

 

I would like to see a sedan with 18x9.5s on it.