what size of joist would i need for a room size of 4.828m x 5.500m?

Answer 1

12" joist for a 12" span? Good God.
5.5M is getting pretty big for a board. An engineered joist might be better if you're going to have a room on top.
It depends on how close together they are and how you fasten them together. A real tight connection between joists can help them support each other. 2x12s would make a fairly stiff floor at 17' and 16" apart.

Answer 2

Depands on whether you have any dwarf walls i.e. small walls under the floor which break the span of the joist , the more dwarf walls you have the smaller cross section of joist you will need.Also need to know which way your joists are spanning are they spanning 4.828 or 5.500m , also remember to add at least 150mm end rest for your joists.
Hope this helps.

Answer 3

The traditional calculation is one inch per foot; a twelve-foot span would require a twelve-inch joist. Modern materiels may have reduced this ratio, but better to make it too strong than too weak.