You need to know the dimensions of the profile used for the lining/casing - it's generally at least 30mm thick (or for a rebated casing at the rebate) , but that does vary (for example the oak casings I was installing yesterday were 50mm thick at the rebate). Then to that you add three x 3mm gaps across the width, and two x 3mm in the height. 3mm is the standard gap around 3 sides required for fire doors, and you have two doors which share a middle gap, plus you also need to allow at least 3 to 4mm gap at the bottom above finished floor level (if it isn't a fire door you can have a greater gap, but the bigger the gap the more chance of draughts in winter) Additionally, you normally allow an extra 15 to 20mm in the width of the opening for alignment (just in case your framing isn't bang on plumb, is warped or bowed, etc), packings, etc - it is almost impossible to get a door casing into an exact size hole, so some "shimmy space" is necessary, and gaos can be packed with mineral wool)/foamed afterwards. You add an extra 10mm in the height for the same reason (on double door linings/casings you should also add screws through the heads to counteract any tendency of the heads to droop)
So for 2 no 1981 x 686mm (6ft 6in x 2ft 3in) doors you'd need at least:
10 + 30 + 3 + 686 + 3 + 686 + 3 + 30 +10mm in the width (1461mm)
and
3 + 1981 + 3 + 30 + 10mm in the height (2027mm) plus the thickness of your floor covering (e.g. carpets)
So an opening in the studwork of around 1465 wide x 2030 high for a pair of 1982 x 686 doors on 30mm thick linings, but do check what your supplier states as the dimension of their lining/casing kit
The 3mm gap is fairly standard for fire door type hinges. Thinner stamped metal hinges may require a slightly thinner gap, but 3mm is a reasonable figure to aim for as you will lose a bit of that when you paint the doors
