I have been to Tunisia many times, staying in various hotels in Sousse and Port El Kantaoui, and have never seen a 'single room' as such and don't think they have them - they all seem to be full doubles, with two beds, which gives you plenty of space. I visited a friend in the Tej Marhaba last week and her 'single' room had a king sized bed.
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I've stayed in a 'broom cupboard' sized single room in other countries though and it is horrendous that they can charge extra for them.