Room on buses for luggage?

See:
http://www.postbus.at/en/Planning_your_trip/Travelling_with_luggage/index.jsp

*What is hand luggage?
Objects that can be stowed above or underneath one’s seat or kept on one’s lap without disturbing, bothering or endangering other passengers are considered hand luggage. Hand luggage is carried free of charge under the responsibility of the passenger. When a lot of people are travelling on a vehicle, hand luggage can also be stowed in the luggage compartment.

What is luggage?
Luggage is anything that due to its size may not be brought into a passenger compartment of a bus as hand luggage. Whether luggage can be transported in the passenger compartment depends on, among other things, the number of passengers travelling on the bus. The transport of passengers' luggage is subject to the payment of the respective applicable fee. The drivers have the right to refuse the acceptance of luggage if there is not sufficient space available for the proper stowing of it.
If any luggage cannot be transported on the same vehicle for which you purchased a ticket you have the right to cancel the journey and to request refund for your ticket.

What am I not allowed to take with me?
Excluded from the carriage on Postbuses as hand or other luggage are the following objects:
Objects with a unit weight exceeding 25 kg
Objects that cannot be loaded because of their nature/size
Objects the content of which comprises hazardous or explosive substances
The driver shall have the right to examine the content of luggage in the presence of the respective passenger if it can be reasonably assumed that one of the above mentioned reasons for exclusion apply*.

Or corresponding by e-mail: service@postbus.at

Images of Austrian Postbuses:
http://www.transportimages.com/buses/buses/busesinaustria/austrianpostbus

So yes, your airline-carry-on-sized suitcases can go in the under-bus luggage storage area, which is pretty large on most inter-city buses. I expect that cases of the cargo area filling up on that bus are rare, at least outside of ski season.

Note also, that the bus from Hallstatt to Bad Ischl does not leave from "downtown" Hallstatt, but from Lahn, which is less than a km walk along the pedestrian street that goes south from Hallstatt. I didn't find that to be a problem (I was staying in Lahn, anyway), but you should take that into account.

And, BTW, I had no problem taking my less-than-regulation-carryon sized luggage onto the bus with me.

Thank you Lee! Our intent is to take the train from Hallstatt to Bad Ischl, and then to catch the bus in Bad Ischl that goes to Hallstatt.

"Our intent is to take the train from Hallstatt to Bad Ischl, and then to catch the bus in Bad Ischl that goes to Hallstatt."

I think you mean "... that goes to Salzburg". I've traveled by bus from Salzburg to Bad Ischl and on to Hallstatt. I have a slightly-less-than-regulation-sized convertible bag, and I was always able to bring it on the bus with me on the bus.

Hahah yes Lee you're right, that was a typo! Thanks for the reassurance :) And has anyone had trouble actually getting a ticket on the bus, since you can't really buy them ahead of time? Any issues with the bus you were trying to take filling up? We'll be there in 2 weeks.