Most power supplies are the same physical size, and fully interchangeable. Fairly old ones are to an AT standard, new ones are ATX standard and have different power connectors. You will probably get a power supply with a higher power rating than you have now - this is not a problem though - in fact it's a "good thing".
Just to help you check, a "standard" power supply is around 14x8.5x15 cm - and attaches to the csae, at the rear, by four screws.
An AT power supply attaches to the motherboard with a two piece connector, which has a single row of wires - an ATX has a single connector with a double row of wires.