Pouet's Tiny Intro Toolbox Thread was one of sizecoding.org's main inspirations.
The Hugi demoscene diskmag has been 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, Řrřola, 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 register starting values. See also DOS Register Starting Values to understand why the register starting values for .COM files are set the way that they are.
The Sparks And Flames x86 Instruction Chart is awesome, once you can "read" it. Find out what the bright green (1-byte) commands do, and use them. For the extreme sizecoding (<=32 bytes) it's also very useful to have the HEX value in sight.
The Geek Edition x86asm-ref is comprehensive and dense. It might be the only reference you'll need. The last part covers modr/m bytes, which is useful.
Rene Jeschke's preservation of siyobik.com's x86 Documentation is one of the most compact and understandable references to the x86 instruction set you'll ever see.
Robert Collins' Undocumented Opcodes page has some gems you can exploit depending on your target environment.
PC Speaker Basics For everything from simple bleeps to sophisticated synthesizers.
Program Segment Prefix examples what's in front of our .COM code, and why.
Never underestimate The Hidden Power of BCD Instructions.
Programmable Interval Timer: Timing is everything, if you're targeting a slow system.
BIOS data area: Direct system communication overview. There are environment values here you may be able to utilize.
Summary of MIDI messages : Summary of MIDI messages
Online IEEE 754 Converter Excellent tool for optimizing/reusing FPU constants
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.
HardCode is a repository of 7800 64KB and smaller intros.
256b.com is a website dedicated primarily to 256b and smaller releases.