WC Extractor Fan - Smallest ducting possible?

[SkipFeeney]

Hi,

I am in the process of putting in a downstairs WC. To put in an extractor fan the ducting will need to go through 2 walls.

Is there any ducting that is very small so i don't have to make two huge holes?

Can I use this ducting:
http://www.screwfix.com/p/manrose-rectangular-flat-channel-1000mm/14118

Also what vent for the outside wall will attach to this ducting? or will i have to convert back to a cylinder ducting.

Thanks

 

[muggles]

Ducting needs to be sized according to the length of the run and the power rating of the fan