Replacement Boiler Size Calculator
Using the above you don't need room sizes, just outside wall sizes.
If you oversize it, use all the worst window and insulation choices, pick Scotland as location, it comes in at 40kw.
To get 100kw you need a 20mtr square, 4mtr ceiling, 2 storey metal single glazed, flat roof building in Scotland with 15kw allowance for water.
Using the above you don't need room sizes, just outside wall sizes.
If you oversize it, use all the worst window and insulation choices, pick Scotland as location, it comes in at 40kw.
To get 100kw you need a 20mtr square, 4mtr ceiling, 2 storey metal single glazed, flat roof building in Scotland with 15kw allowance for water.