Kitchen Extractor Fan Question

[littlykitty]

My Kitchen has an extractor fan on the celling that measures 8.5" x 8.5".
It is wired into the electrics, and there is a electric switch next to the light switch to operate the fan.
(It came built in with the house [The house is less than 5 years old])

It is not very good at sucking out the air.

I want to buy a better one but have two questions:
1) What features do I have to look for when buying an extract fan?
2) My fan measures 8.5" x 8.5" but I can seem to find any that are that big. Most fans are 4-5". Am I missing something?
I want one that is the same size as the original so I do not have to make the hole in the celling bigger or smaller.

 

[PrenticeBoyofDerry]

The room size will be a factor, you need to measure the cubic volume of your room. This will be multiplied by the rate of air flow in the room. Kitchens tend to be 10-15 times per hour. This can be given in cubic foot or cubic metres.
This will give you the minimum capicity the room requires, also as it is ceiling mounted, centrifugal extractor fans should be considered as it likely to be installed in to long ducts.
As for your hole size, it seems a bit on the large side.
What make/model is currently installed and what size duct does in run in to?

 

[foxhole]

The external dimensions are not important what is important is the hole size, it's probably a 6inch fan, easily replaced, though giving it a good clean first may improve it's current performance.