Over the years, MrFrizzy and I have used the "supermarket dinner" to save on (occasional) meal costs in Europe. Many supermarkets in Germany and Austria have hot food at their deli counter, but even if they don't, you can put together a perfectly tasty cold meal of salads, beans, fruits, ham/sausage, you name it, to enjoy either as a picnic or in your hotel. And as for portion size, you generally don't get "too much" at restaurants in Europe. Should it nevertheless happen, you can always ask for the excess to be packed up and taken home. And for eating in a restaurant, do try the daily lunch specials, usually advertised on a board on the street. Usually good value and seldom too voluminous. Oh, and it's not frowned upon at all to share a dessert.