RE Technology:
There are two basic types of filtration you've touched upon there; filtering for biological contaminants and filtering for chemical contaminants.

Think of Ceramic and Glass Fiber filters as screens. Biological contaminants have a minimum size and the pores in these two types of "screens" are smaller than those contaminants so they are unable to pass through the filter with the water. Note that viruses due to their extremely small size can still pass through many of these filtering elements.

Several companies are also testing silver in particle filters because it seems to be able to kill the biological contaminants. I don't know much about its effectiveness.

There is also the chemical treatment option to kill biological contaminants, which either use Iodine or Chlorine. Since we're talking filtration, I'll skip those.

Granulated Activated Carbon on the other hand filters chemicals out of water (Toxic Contamination). Carbon by itself will absorb a very large number of different chemicals, and activated carbon has been electro-positively charged making it "attract" even more. It's granulated to offer more surface area IIRC.

RE Nasties:
To quote from ETS: "Most bacteria are larger than 0.5 microns in diameter, but some are as small as 0.2, though the ones most likely to cause problems tend towards the larger size."

Viruses on the other hand are a whole other ball of wax depending on type. They can be smaller than most filters can screen. However, Iodine and Chlorine can kill viruses. Read the link to ETS above for more info on this.

RE Capacity:
Yes, capacity generally refers to the amount of water that can be safely filtered before the filter must be cleaned or replaced.

RE Size:
This one is really up to you.

I carry a Katadyn Pocket Filter when I go camping, which resides in my small BOB. It has a .2 micron silver impregnated ceramic filter a capacity of 13,000 gallons and will filter 1l/m. I also have a Katadyn Inline Carbon Cartridge to supplement the Pocket Filter (200 gallons before GAC must be replaced). Together they can filter just about everything nasty out of water except for viruses. I could either pretreat or post treat the water with Iodine or Chlorine to take care of the viruses. I will admit that I don't do this though. Thus far, knock on wood, the Katadyn Pocket and Carbon Cartridge have been enough.