What size vehicles look good with Standard gauge?

At the risk of irking folks who like short answers, I will claim that the answer varies.  1:32 is a good start, but some larger vehicles work well, too.  1:24 is probably too big for many, but others use them.  1:28 is a great size if you can find any.

That said, the effect you want to achieve helps guide the answer.  Do you want to recreate what might have been on the floor of a 1910s - 1930s boy's room?  He was no respecter of scale, and a wide range of smaller and larger vehicles could probably make their way onto his floor.  Dimestore varieties of the time would have been mostly tin or cast iron or slush, and you can find all kinds on that evil auction site.

Or are you looking for a "hirail" Standard Gauge effect, sort of in the vein of Tom Snyder or Richard Kuhn or some others?  If you are running Lionel #33s, #8s, #10s with 500-series freight cars and 300 series passenger cars, stick with the 1:32 and toys close to it.  If you are running #5s or #6s or #42s with 10 series cars or #402/408s, #400s, or #381s with 200 series or 400 series passenger cars, larger vehicles will fit better.  You can also fit the vehicle to the context, such as HO-sized cars parked in front of the villas and bungalows, 1:32 parked next to the power stations and 113 series stations, and 1:24 parked next to the 121 sized stations.  (I guess you could even use 1:18 next to the gateman, and 1:12 next to the flagman .)

Since most of my trains are on the smallish side (except for the Flyer), I have some 1:32 and some 1:28.  I am eying repos of the Paya tinplate taxis as a possible addition because, really, I am not too interested in sticking to "hirail."  So I guess I don't fit either category.  and others could be envisioned.