How much luggage for a semester abroad?

One bigger suitcase to check. Keep in mind the weight limit for that bag. One 21" rollaboard or backpack to carry on (which you will use for your side trips). If you are concerned about bringing more home, you can always get a cheap nylon bag there and check that second bag coming home. You will have to pay the excess baggage fee, but I wouldn't take an extra bag going over unless it is a nylon foldup bag that can be packed inside your suitcase (e.g. something like http://travelstore.ricksteves.com/catalog/index.cfm?fuseaction=product&theParentId=8&id=38)

My son just did study abroad Spring semester in St. Petersburg. He used a Rick Steves convertible backpack for his carry-on, and his checked bag was his footlocker, which met the required dimensions. We were careful to pack it to under the 51 pounds BA allows. We had an additional issue as Russia has an import limit of something around 77 pounds, with a nasty per pound (kilo, actually) surcharge at immigration, probably before one can get to an ATM machine. He had no problem going in, and none coming back in spite of additional materials he picked up (among the travel there, he and friends went to Istanbul during their spring-break week).
This is a once in a lifetime opportunity - enjoy all the aspects of it that it offers!

I feel a bit differently from many other posters here. Lots of people say they can travel for any length of time with just a 21 inch suitcase, but I can't. And for a full fall semester, I think that will be much harder, because (as you say) that encompasses lots of different weather. The needs of someone actually living abroad are different than those of a short term visitor. So, I'd plan to check a 24 inch suitcase and also take a carryon bag. If you buy souvenirs, you may need to check a second bag on the way home (you can buy a cheap suitcase in Vienna if you need to). But you may not buy that many souvenirs; wait till you see the prices. And I don't think it's so terrible to have two suitcases on your return; I don't think it's "too much." Do be aware that you will probably have a fee of $50-100 to check a second bag (so add this to the cost of any large souvenirs). The above does NOT mean I am endorsing taking lots of unnecessary stuff. Do plan on buying some things there to supplement what you bring, and don't bring anything you can easily buy there. For example, only bring starter amounts of toiletries, and only bring a few (at most) of any one kind of item, like sweaters. Also, don't bring any hair dryers, curling irons, etc; just buy new ones in Vienna. A non-travel one won't work in Europe, and there are lots of reports here that the travel ones don't get hot enough. But do bring the computer stuff you will need; it's all available in Europe, but at higher prices.

Semester abroad-- you need a big suitcase for getting your stuff over there, but watch the weight, and a carry on for weekend travel. You can also pack a folding duffle inside one of these or a backpack.

Suitcases are second. Stuff is first. Figure out what you need for ten days (or whatever the interval is so that you're not dorking up your studies by doing laundry too often). Toss in the jackets and other crap that girls seem to need. See where it fits. Trim if you can see a way. Go with whatever it takes. You'll only have the going and coming with the full shebangus. You'll figure a way to move it around when you have to. If push comes to shove, you can tug two big rollers and strap something to your back. Do what Bets said about the empty extra.

Congratulations! Local woman makes good. Can you plan on buying some of the clothes you need when you get to Vienna? Here's a tip. Behind the Ibis hotel in Vienna and across the narrow street there is an Italian restaurant on the corner. The cook there taught my wife how to make his style of pizza. The food is really good. Linz is another Austrian city that is interesting to visit. The torts are famous and there's a neat farmers market on Thursdays, I think. One checked suitcase and one convertible carry-on plus your purse should get you there and established. Remember you are going to have to move around taking these pieces of luggage by yourself.

What is the advice from your study abroad office? When our son spent a semester in Madrid, the study abroad office recommended clothes for about one week to ten days. You will find that the students will want to buy local clothing to match local style, and fit in better. We found that to be very true. In fact when we visited at mid semester, he asked us to bring an empty suitcase so that we could carry home most of his original clothing. We did. When we met him at a cafe in Madrid he looked just like all the other European students. When he was ready to return he sold some, gave some away most, and carried the rest home in his original one 28" rolling bag which had lots of room for his souvenirs. We also took some of his early purchase back with us at mid semester. Also, you could ship some of your clothing via the local post office and carry the good stuff with you on your way home. But he is advice to many fellow students later was take less than you think you will need.

Andrea, check your Profile for a private message about the Lewiston/Clarkston/Moscow/Pullman etc Travel Group meeting!
(aka the "Towns & Villages near Moscow Travel Group")