What size joist should I go for?, one book I read says I can use 100x50mm
(4x2) but I`m not sure.

A 47mm x 97mm joist on 400mm spacings with SC3 timber and 0.25-0.5 kN/m2
allows a span of 1.92m, making it just compliant with building regulations.
If you had wanted a bath installed, you would double up the joists
supporting it.

If your intended flooring weighs more than 0.5kN/m2, you'll need 122mm (5
inch) joists.

A sleeper wall or thicker joists would improve rigidity, if required. If you
install one, ensure ventilation is adequete. In fact, you must ensure
ventilation is adequete anyway. Ensure you install good insulation and
airbricks. Take the time to install underfloor heating for a new floor.

Christian.