In addition to what's below:

The available tire sizes on the 718 Boxster S when you buy it new are 19" and 20", but Porsche sells a winter tire / wheel set for the 718 with 18" wheels.

What are the advantages and disadvantages of the different wheel sizes? I imagine the 18" wheels will be less prone to damage in pot-holes and such, but is there any issue with clearance around the brakes and such? I suppose not, given it is a Porsche offering, but I am just interested in the tradeoff and puzzled about why they would not offer the 18" with summer tires as well if they offer it with winter tires.

Get the 18's - more sidewall to smooth out the ride offered by winter roads. And in winter, narrower is better (assuming you're driving in snow)

I have 18" Porsche wheels on my 981, without any problems at all. I can't recall the tire size- it's whatever Porsche put on the wheels at the factory. My car came with grey 20" wheels which are gorgeous, but not enough sidewall for our bad local roads.

For tracking, MikenOH has 18's on his 981. And that's most certainly in the summer.

Quote
jeff in VA
The available tire sizes on the 718 Boxster S when you buy it new are 19" and 20", but Porsche sells a winter tire / wheel set for the 718 with 18" wheels.

What are the advantages and disadvantages of the different wheel sizes? I imagine the 18" wheels will be less prone to damage in pot-holes and such, but is there any issue with clearance around the brakes and such? I suppose not, given it is a Porsche offering, but I am just interested in the tradeoff and puzzled about why they would not offer the 18" with summer tires as well if they offer it with winter tires.

18" tires are less expensive to replace and will work perfectly fine driving on the street. As Roger said, more sidewall to add a bit more cushion when driving on rough roads. Stock 18' sizes are 235/45/18 F and 265/45/18 R.
When my Nitto NT01 track tires are done I think I'll move to the RE-71 in these sizes.

The 19" rims that came stock with our 981S are fine with the firm X73 suspension.

1. smaller wheel/tire combos are lighter, which is better
2. not all 18" wheels clear brake calipers the same. The spoke design and shape matters as well, as does the offset
3. i prefer the "feel" of a tire with some sidewall, since it has a progressive give in turns.

plus, obviously, you will likely damage very low profile stuff, and the ride is poor.

I run 17s on my 986s (track and street), except for winter when i run 16s

I think i would go 18 on a 981.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

I think the trend has just pushed progressively larger to keep up with style/image for these premium sports cars. In the original 986 (20 years ago), they started with 16" although many just used them for their winter wheel/tire setups. 17" was standard with 18" as an upgrade. Later generation models had the option of 19 inch wheels, and the suspension was updated to allow for these larger wheels to not punish the driver. I think the 986 model range would have a really uncomfortable ride with 19 inchers, but the later models not so much. Now, 20s can be used. You lose some comfort and risk wheel damage with these larger wheels, and the tires are more expensive, but you may gain some ultimate grip, and they look "cooler" for braggin rights. For a 981/718, I would say you could do 19s easily without worry of a too harsh ride, but not the 986. It seems the factory wants you to run 19s, with 18s only for winter use. I'd go with the 19s on the 718.

I have 18s on my 986, as I much prefer the look to 17s. My car looks funny with 17s cause I have side skirts, and they don't fill the wheel wells right. The ride does suffer a bit, but I can live with it.