Resources
Discussions
Pouet's Tiny Intro Toolbox Thread was one of sizecoding.org's main inspirations.
References
The Hugi demoscene diskmag has been very kind to the sizecoding movement. They included many programming articles since it started in 1996. Additionally, the Hugi Size Coding Competition Series is the gold standard for learning how to sizecode: Each competition was given a specific task, then participants submitted their results. (All of the entries with their source code are available, as are the rules for each challenge.) Hugi #35 also provided a 256b intros round-table between Baze, Digimind, Rrrola, Pirx, Optimus, and Seven.
fysnet.net has an extensive list of DOS .COM register starting values. If your environment isn't listed, a program is also available that can print out all of the starting register values.
Repositories
Tinyprogs are so small that you sometimes don't need the source code to understand them: Just pop them into a disassembler (like IDA) or a debugger and look at them. If you want to grab specimens for study, here are some places to start.
Pouet can be searched for the latest tinyprogs: 256b, 128b, 64b, and even 32b and smaller.
HardCode is a repository of 7800 64KB and smaller intros.
256b.com used to be a website dedicated to 256b releases, but has been down since 2009. A mangled archive is available at archive.org.