Here is a sample build taken from a thread over at AVS. This build is sorta buried...its like on page 38 or something
http://www.avsforum.com/avs-vb/showthread.php?t=1302559
CPU: 2 choices here. Both are enough for typical HTPC use. Both are low wattage as well. The E5200 is harder to find and out of stock at Newegg but I prefer it over the E3400 slightly if you can find it. Both CPUs are more than enough for playback. Idle wattage is only 30-40 watts.
1. E3400 Wolfdale 2.6 GHz Slot 775 - $51
http://www.newegg.com/Product/Produc...82E16819116348
2. E5200 Wolfdale 2.5 GHz Slot 775 - $54
Motherboard: ASUS P5G41T-M Micro ATX - $60
Great board for HTPC. Has optical output. This motherboard takes cheaper, more efficient DDR3 RAM.
http://www.newegg.com/Product/Produc...82E16813131616
Graphics and Sound: Sapphire Radeon HD 5450 Fanless - $26 AR
Since Wolfdale doesn't have built in HD Graphics you'll need one. This will play and bitstream all HD video and audio formats. Has HDMI. Quiet without a fan and idles at about 6 watts of power.
http://www.newegg.com/Product/Produc...82E16814102922
RAM: Since this is a super budget build look for a sale on 4GB RAM. This can be found for about $35. I'll post my "go-to" RAM for reference though.
http://www.newegg.com/Product/Produc...82E16820231190
Hard Drive: The size is up to you but something like the Samsung F4 2TB is what I recommend. This drive can be found frequently on sale for $80.
http://www.newegg.com/Product/Produc...82E16822152245
Case: As stated in my first post of this thread this is a personal choice. Since this is a super budget build I think we should target $50 for the case and PSU.
So that's $172 for the CPU ($51), Motherboard ($60), RAM ($35), and Video Card ($26).
Add in another $80 for a huge 2TB drive and an additional $50 for a case and PSU and that puts you right at $300 total for a very capable HTPC that's pretty low wattage as well.
Doesnt include a blu ray player is the only thing. but you can always add one later if wanted.
As for size, micro-atx is fairly small. My htpc is slightly bigger than my onkyo reciever. If you look at that AVS thread you can see micro-itx (think thats the name) builds which are super small. Though those are a bit more expensive i think