Upload & Sell: On
I can recommend the 18/3.5 ZE as being a very sharp lens that is excellent for landscapes, but it's not small, by any means.
I've attached a comparison of some well known 17mm and 18mm Alt lenses. I've owned all except the Oly 18/3.5. The Tamron (newer model 151B with no built in filters) and Tokina are both good, and the Tamron is quite a bit smaller. I've also owned the SMC Pentax 18/3.5, buit that's not as good as the Tamron and Tokina, by a long shot.
The 82mm diameter filter for the Tamron is kind of unusual, as it fits in the outer part of the OEM hood, not on the "filter mount" threads at the front of the lens (which I think is 67mm). The 82mm filter for the 18 ZE is because the front filter size is 82mm.