There are a number of things to keep in mind when making terrain whether it be for your gaming table at home or your local club. I think the single most important thing is usability. Looks are a very close second, but if it can't be used in a game and add to the quality of the game overall, what's the point?
The cathedral above was a piece I did a while ago. It's the biggest piece I've ever made coming in at two feet wide by four feet long. That leaves one foot of space all around it if you place it in the middle of a regular sized board. Most importantly, it's usable.
This particular piece is not without its flaws though. Because of it's size, it can be a bit unwieldy to use and work around with other terrain pieces. It can monopolize a table quickly. It could use a few more details as well to bring it to life. But... those are secondary things.
How do you make terrain usable then?
It's easy to explain, but hard to do when it comes to construction. You need to be able to place models in and around your piece. You want models to be able to interact with the terrain during the game. You want key areas to be wide enough to accommodate certain size models. You want there to be places to hide behind and get partial or even full cover from the enemy. You don't want to have to worry about models always falling over or getting damaged during a game.
Putting that into reality can be a different story. The biggest thing I do when building any kind of terrain is to have my army handy. Test out your terrain as you build it. Will a model be able to stand on the steps leading up to the second floor without falling over? Can you actually hide your dreadnought behind that wall? Can a tank fit through that particular opening in a wall?
Set your models on there and see. Test it out as you build your piece. You may have to make some minor adjustments here and there to accommodate what you want, but it's well worth it in the end. Does making a terrain piece usable have the potential to create some odd looking shapes here and there because you had to elongate or widen a particular feature... possibly. But the enjoyment you'll get when you can actually run your dreadnought up the wide staircase to the top of that altar of doom is well worth it.
Make usability matter from the beginning and build for it
Start with usability in mind in the beginning and work towards that as you actually construct your terrain piece and you'll come out much better in the end. Don't just build something and set the final piece on the table only to see if it works then. Set yourself up for success from the start. You'll find yourself using terrain more and having it add to your games instead of being a constant source of frustration.
Make sure to check out these posts as they might help:
Cities of Death Cathedral terrain piece (3 part series)
Is defined area terrain a must have game mechanic?