I like nerding out on forms and content construction.
Two books that have been foundational to my understanding of layout and content design:
- The Elements of Typographic Style by Robert Bringhurst
- Thinking with Type by Ellen Lupton
If you just bought those two books and browsed them for a night, you would have all the tools you needed to build very accessible forms, even if they're long.
There are a few basics here. I'm not referencing specific studies below for the same reason building a technical manual with 20 pages or with 2,000 pages doesn't require (outside of a longer index and ToC) a completely new set of science.
There are many ways in which order, hierarchy and relationship can be visually established. You've pointed out a few of them, but I'll just run through a list quick:
- Size: Something bigger is more important than something smaller.
- Adjacency: Things close together are related. Things far apart are unrelated.
- Pattern: Things that are organized are related.
- Color: Colors in similar families are "related," while colors in different families are "unrelated."
- Saturation: More-saturated is more noticeable than less-saturated. You notice the key to each of these list items before the description of each.
- Order: This list is one object because it has a continual sequence.
It's not exhaustive but gives a number of tools to play with. Combining different aspects can have interesting results, so its fun to experiment.
Your specific questions:
- What is a header? A header is a word or short phrase that describes a grouping of content. It is spatially adjacent to that group of content, separated ideally by dead-space from other groups of content, and is visually distinct from the content it describes (size, color, saturation).
- Which text should be bold? Bold text should be used sparingly. Text that must not be missed should be bold.
- What size should different text elements have? Text elements that are equivalent should be the same size, color, saturation, and pattern.
-- section and page headers
-- section descriptions
-- form box labels
-- form box content
-- column headers
-- table cell content
-- secondary content (sidebars, tool tips, etc.)
Some specific questions:
- Do your users report having issues understanding what a certain section of information is? Improve the descriptive quality of headers, and either increase white space around them, make them darker, or make them bigger.
- When you say "making the interface easier to predict," do you mean making it easier for the user to know what field or piece of information is required? If the user reports or is observed having problems telling required and optional information apart, make required information stand out. Color/saturation and symbols that make something stand out are expected by the user when a piece of information is required.