We liked staying at Fernandes´s guesthouse. Close to the Rossio square and metro stations - about 5 minutes walk downhill, a bit more when returning in the evening as it is quite steep uphill walk. Take into account that all Lisbon is walking up and down :).
The room was small with one roof window but suited us well as we didn´t spend much time indoors. The common rooms are nice, beautiful view from the living room, well organized kitchen, house rules and suggestions written on the walls. Host Mariana was helpful and responded very quickly. Wish you all great stay in Lisbon!