I found you have to mix and match vehicles and buildings to make a scene look "just right". Sometimes it works and sometimes, well, you make do to set the scene you want. These photos were taken on my last layout.
The Good Humor Ice Cream truck with the Dalmatian guarding it looks fine since it's backed up next to a customized Lionel Rico Station. (The Dal looks fine size-wise, too, since we had an 85 pound Dalmatian, Mick ,who looked just like this figure except he had white fur, not black, on his tail.)
The two pickup trucks in the photo below, which cost a buck apiece in Dollar General, look good on the Starlite Diner's parking lot because the spaces are pretty short. The Hummer on the right side looks alright since it's next to an RV. The red car in the center didn't last in that place too long as its size didn't look right after I took this photo.
Buses have been a problem for me since so many seem to come in 1:50 instead of 1:43 or 1:48. When you love the look of Greyhound's Scenicruisers, you just shrug and say, ok, put them towards the back of the layout and get on with life.
But when you find a Worcester Bus Company bus and your mom's from Worcester, you just HAVE to find a way to make the scene work. In this case, it was on the back of my last layout along with the Trailways bus from MTH. (Since my maternal grandmother used to ride the Trailways bus from Massachusetts to NJ to see us, you know I just had to find a way to make that work with the Worcester bus, so the under size issue was something I had to play with a bit to try and improve things. Setting it by the chapel helped a bit.