The Sagulator is a good quick calculator you can use to work out the type and thickness of material for any shelf or tabletop spans. It also factors the type of material into the load-bearing capacity. Consider the maximum weight any load-bearing horizontal member is likely to hold, and confirm that your design will be able to hold that much without too much deflection.
Also consider static loads (e.g., putting a heavy box on a shelf and leaving it there) vs. dynamic loads such as people dropping things on top of the structure, leaning on it, jumping on it, etc. Static loads can warp a shelf or table over time, as you probably know if you've ever stored heavy books on a particle board bookshelf. But on the other hand, dynamic loads can involve someone plopping heavily into a chair, in which case you need to be able to support significantly more than just the weight of the person--like when you first step onto a bathroom scale and the reading zooms way past your weight before settling on your actual weight.
The tried and true method is to look at other pieces similar to those you want to build, and use those measurements as the starting point for your own piece. Aprons and other reinforcements can add a lot of strength. You can always beef up load-bearing components, but also remember to consider the relative sturdiness of the materials you're using vs. the materials someone else used. You can plug both materials into the Sagulator to see how they compare, given the same dimensions.
