The author gave weight only for one speciment, a 2.2 by 2.2 meter sheet weighted 4.2 kg. This means that it weights around 900 grams per square meter, a bit more than my blankets weight (they are around 750 grams/m²). However this is only from one sample and usually these bed sheets had between 10-20 warp yarns per centimeter (21 being maximum) which might make them lighter. For comparison a typical Finnish Iron Age outer wool clothing might have had around 15 warp yarns per cm.They don't look like real blankets to me, Martti. Not as thick and fluffy, but more like a flanel kind off sheet.
Some of the sheets were also washed several times with soap to make them fluffy (i.e. "sarka" cloth).