You will find tables of acceptable joist sizes in Appendix A of
Building Regs Approved Document A 1992 edition at

http://www.planningportal.gov.uk/upl..._AD_A_1992.pdf

note the tables are for good quality - ie graded - timber.

Also note that the sizings given are for an assumed "live" load on the
supported as given in the text of the regs (effectively so many Kg/
Newtons per square meters) - thiat is the assumed max load of the room
contents + passing traffic. The tables allow in addition for the self
weight of the joists.

HTH