Rockershovel, while some larger locomotives will negotiate 027 curves (and particularly clear the 027 switch box housing), I personally tend to stick with engines that look good doing it.
I've always had small layouts and so 027 track (with 027 curves and switches) has been a necessity. And at one time in the past, I too was looking for the largest locomotives that would clear the switches and make the curves. I had one of the early MTH Railking Dash-8 engines. And while it did make the curves, it looked out of place on my smaller layout with more traditional 027 rolling stock.
Here's a link to a useful website. You can scroll through the categories (it's a "sensitive" drop down menu) or you can type in the number of your locomotive in the search box on the upper right. The product descriptions will tell you if it is an 027 or 0 gauge steam engine.
http://postwarlionel.com/motiv...055-santa-fe-hudson/
As far as modern engines go, it'd be good to go through Lionel catalogs before the introduction of FasTrack. Engines that came in starter sets before FasTrack came with 027 track, so that'll help out. One engine not mentioned here yet, is the K-Line starter set Pacific type 4-6-2. Like with some Lionel steam engines, the body casting is slightly smaller and doesn't look out of place with smaller rolling stock.
If you really want to get ambitious, I chopped down the size of some of my 027 switches, mostly for the option of having the switches closer together for my switching yard. You can also cut down the corners of the switch box housing if you have any items that are rubbing against or hitting the switch box housing.
For me personally, I like the look of smaller steam engines on my layout. Lionel during the MPC years, came out with some new and interesting smaller plastic body steam engines. I add weight to mine so they pull better. The plastic bodies make adding details and repainting easier. Even just repainting them to take away the plastic-look of them is such an improvement.
Finally, here's a video I have always liked. Maybe you've already seen this one and the one below. You will see some larger engines (no steam though), clearing 027 switches, though there are one or two 42 inch radius switches. That older style Williams GP38 diesel is pretty big and I'm impressed that it cleared all the 027 switch box housings. There's also some larger rolling stock, like a Weaver shorter covered Borax hopper. While the length of that car is shorter, the width of it is wider than the typical Lionel car and yet it clears.
I guess it would always be good if you are unsure, to ask a question here if anyone has such and such an item, and runs it on 027 track, before you actually buy it.
And just for fun, here's another video showing postwar steamers and diesels that will run successfully on 027 track and curves.
