If you have a big enough saw the only limitation to the size of a beam is the size of the trees. We have access to a horizontal LBL Brenta saw which will cut lengths up to 18m, so beams of 1.2m x 1.2m x 18m are possible. Lengths from 1m to 6m are easily available. Lengths from 6 to 8m are less common. Lengths over 8m are difficult, but possible.
Beams are priced by the cubic meter, but the rates go up the longer the pieces are, usually in 1m length/price brackets.
Standard sections are:
Sections under 100mm or over 300mm are available from stock, attracting a 10% premium