Sorry if this is a repeat question, but how deep and wide would footings have to be for a porch 2m deep x 2.5m wide? Thanks in advance for any advice
It depends...porches are not subject to building regs so minimum depths do not apply.
However, if you want to comply with regs then you need to be at least 450mm to the base in gravels and 0.75-1.0m in clays depending on the soil type, or deeper if you have trees nearby.
Drain pipes are another consideration as you will need to ensure you are 150mm below invert level, regardless of building regs.
Width wise, 450mm is an ample minimum.