I want to try fish (either fillets or whole) baked in parchment or salt. I have found several recipes but they specify a certain weight fish. How do I adjust the cooking time for the size fish that I have?
The baking time should vary with the thickness of the fish (or anything, really), not with the weight. This is because the heat is coming from the outside, and has to conduct through the fish to get to the center. Of course, for the most part, a heavier fish is also thicker, so a heavier one takes longer to cook.
One option is to use a food thermometer. Cook until the center reaches your target temperature minus a few degrees (the center will continue to heat even after you take it out of the oven, as the temperature inside the fish evens out). The USDA recommends 145°F for food safety, but often lower temperatures are used (many people consider 145°F overcooked).
Another option is to cut the filets to the size expected by the recipe.
If you want to try and estimate the time, the general rule is that heating time follows (approximately) an r² law, that is, twice the thickness, four times as long to heat through. Note that different materials heat differently, so two different species of fish may be slightly different. There is of course a a formula to compute this, but you'd spend longer in the lab measuring the exact properties of your fish than it'd take to do either of the other methods :-P