What is a good size amp?

I'm curious about selecting the perfect amplifier for my music setup. Can you help me understand what constitutes a 'good size' amp? Is it solely based on the wattage rating, or are there other factors I should consider? For instance, do I need to match the amp's size to the speakers I'm using, or does the room size play a role as well? Additionally, how important is the quality of the amp's circuitry in determining its suitability for my needs? Ultimately, I'm looking for a balanced approach that ensures both powerful sound and optimal performance within my budget.