Problems understanding size


Sorry if this seems like a stupid question , but how is character size derived ? Are all human PCs considered medium size ? The reason I ask is that in many places on the character sheet there is a box for size bonus , but for medium the bonus is 0 , so there doesn't seem to be any reason to include it for PCs ( Unless they're Frost Giants , but they weren't in the rulebook anyway ) . I can't find any reference to size apart from the chart in the Combat section and that seems to just be floating in the middle of the text about something else .
There's probably a very simple answer to this , but I've never bothered with D20 before and the Conan rulebook seems to assume prior knowledge of the system , which is very confusing for a newbie .
Thanks to anyone who can help , and if I seem stupid then please keep it to yourself as there is far too much sarcastic flaming in response to genuine posts on this forum . I'd rather a friendly atmosphere between people with a common interest .
All humans are medium size creatures. So they have no inherent bonuses or penalties due to size. My rough guesstimate for determining sizes is ths: take a human being as your example, anything roughly double that size is large or half thatsize is small. Not exact, but I don't know if it ever states anywehre in the core D&D books specifically how size is determined. This does kind of assume prior knowledge. My opinion is that it wasn't nescesary to put a space for size, but its nice that they did, just in case.....

Size can be important for grappling, because you get +4 for each size category larger than your oponent's . Large creatures also get -1 to hit -1 to DV (dodge and parry i would assume). This represents the ease of hitting a larger target and the difficulty or a larger creature to hit smaller things. Large creatures also get -4 to hide for being rather conspicuous.Also larger creatures tend to have higher str and con and less dex than medium creatures.The formula for max carrying capacity is also increased. Theses bonuses just pile up as you go up in size categories. I'm sure I forgot plenty of stuff, but oh well.

Small creatures (which might be human if they're children) are pretty much the opposite. +1 to hit +1 DV +4 to hide and generally a little less strong. The small races in D&D also have their base movement slashed down to 20 feet.

Hope this helps more than it confuses...................
Thanks a lot guys . I was doing my usual thing of learning how all the parts of the character connect by generating a couple of sample PCs , but ground to a halt over this . Hopefully second ed. will have a more comprehensive run-through of character creation or a page showing the character sheet and explanations of the various sections eg. what on earth is "penalty" at the top of the sheet? So far the only way I've found to learn the system is to puzzle your way through the book which constantly explains one thing by referencing another two chapters on , then re-read the whole book to fillin the gaps , which is time consuming when you want to run it as soon as poss.
Anyway , thanks again , it's most appreciated .
Yeah, the Conan book can be clear as mud sometimes. If you own the D&D Player's Handbook it can help clear up a lot of issues. (of course the systems are still different in a lot of ways) I found that I had trouble keeping the rules between the two straight until I went over them a few times and played a few games of Conan. I think I've almost got the rules down. :D