You're right, RickG. It's not a simple matter of following proportions.
First off, separate baking (especially bread) from savory dishes. Bread is actually closer to having a single approach---providing you're using a formula based on weight. But if your using a volume-based recipe, don't count on it working if you more than double it.
With savory dishes, almosty anything can be halved or doubled with no serious affects on taste, flavor, or texture. After that it becomes a matter of trial and error. And sometimes multiplying or dividing more than that just doesn't work. F'rinstance, I have a great recipe for a seafood cocktail sauce that, among other things, starts with a gallon of tomato product. If I cut it in half, no problems. Cut further than that and it just doesn't work out the same way.
On the other hand, some recipes just don't care how far you divide or muliply them. And, fortunately, with most savory dishes you can taste as you go along, and make adjustments if necessary.
Cooking times, too, may or may not be affected, because the cooking times for proteins are often based on the thickness of the product, rather than the total size. Rule of thumb for fish, for example, is ten minutes per inch of thickness. So, if you double a recipe, using two filets each two inches thick, and pop them in the oven, you don't double the time. Indeed, you don't increase the time at all.
The classic case was a friend who wanted to make roast beef for a crowd. Her recipe called for a 5 lb roast. She wanted to know how much longer it would take to cook two 5 pounders. And was shocked when I suggested about 22 seconds.
Actually, there is a minor adjustment in cooking time under those conditions. But nothing significant. And, as Kuan suggests, a thermometer handles that problem nicely.