Beer is served in a "Maß", which should technically be 1 liter. However, due to the speed that the beers are poured and served at the Oktoberfest, it is likely you'll end up with slightly less.
If your beer is substantially below the liter mark (which is at the line around 5cm down from the top of the glass), you might consider asking the waitress to fill it up properly, which she must do by law.
However, the chances of you being served quickly when your beer is dry the next time around will be substantially reduced if you do. Normally you should just take it as it comes, and accept that you might not get quite as much as you would if you ordered a beer in a normal restaurant in Munich.
Only beers that are brewed in Munich are allowed to be served at the Oktoberfest. The main beers are Paulaner, Spaten, Augustiner, Hacker Pschorr, Löwenbräu and the Hofbräu and all breweries brew a special beer specifically for the Oktoberfest.
Oktoberfest beers are stronger than their normal variety, with most being close to 6%. You should keep this in mind and maybe try to slow down your usual pace to compensate. Some tents will also serve you a Radler which is a mix of beer and lemonade and is a good idea if you feel the beer is hitting you too quickly. If you really want to steer clear of alcohol altogether, tent offer a drink called "Spezi" which is a mix of Cola and Orange soft drink as well as the usual soft drinks and mineral water.
In addition to beer, in most tents there will be people coming around selling Schnapps in little bottles. These are generally a bad idea, as the beer is usually sufficient in pushing you beyond your usual alcohol consumption, but if you feel like you're up for it, you can buy these for a few euros.
Keep in mind that in all tents, you will have to be seated to get a beer. Beer cannot be ordered at a bar, it can only be ordered and paid through the waiters who tend the table you are sitting at. If the tent is full, you might be able to get lucky and get someone who is sitting to order you a beer. Then you can just hang around the tables, although this is strictly not permitted.