Most receivers are the standard 17'' wide and 5-10'' high. It highly depends on the type of computer speakers and type of home theater speakers. Speakers are speakers...they all work the same way. Just computer speakers use different interfaces.
If you find a spot for a receiver, I recommend it highly. It usually gives you tons of inputs, settings, connections and other benefits.
I do not know of any tiny receivers. Receivers have amplifiers in them...that means they have a beefy PSU, heatsinks, transistors and other stuff so they are going to be big.