From my very own experience 1, 1.5 room and 2 room apartments in the BIG 7 cities: Berlin, Frankfurt, Hamburg, Stuttgart, Munich, Cologne and Duesseldorf are high in demand. The reason for that are the increasing number of single households. Berlin is taking the lead on the number of single-households.
Since rentability matters to you as a landlord, you should centre your investment around 1-2 room apartments.
For 1-1.5 room apartments: 25-40 sqm
For 2 room apartments +-55 sqm, if you go bigger than 55 sqm demand will go down pretty quickly.
Built in appliances such as a fitted kitchen will increase your monthly rental income. However, as a landlord you have to calculate the increase in rent according to the product lifetime. The additional rent is calculated as follow: purchase price of the kitchen, divided by product lifetime plus interest. The sum is added to the cold rent of the apartment.
A landlord is installing a new kitchen worth 5,000 euros in his apartment. It can be assumed that it can be used for ten years. If you also apply five percent interest, the surcharge is calculated at EUR 500 plus EUR 250 interest per year, which is EUR 750 per year or EUR 62.5 per month.
Poor insulation or outdated heating systems drive up the additional costs for tenants. Too high ancillary costs deter many apartment hunters, as they not only keep an eye on the rent excluding the heating, but also the rent including electricity and heating.
Tenants prefer large, bright rooms instead of small or poorly designed rooms. If the floor plan of the preferred apartment is not perfect, an expert can determine whether the room layout can be changed with reasonable effort. It's worth it, provided that the property's location and price are right.
Tenants prefer apartments within easy walking distance of a subway or S-Bahn stop. A good infrastructure with supermarkets, cultural institutions and companies that provide jobs make a residential area attractive. A residential area with positive development prospects will still be of interest to tenants in several years' time.