we had the same issue recently with an island hood
extractor fans have a 4"(100mm), 5"(125mm) or 6"(150mm) extract - you have to use the correct size ducting to get the published performance from the fan (it's usually forbidden under the terms of the guarantee to reduce the size of the ducting on the exit from the fan)
Elica, Neff, Siemens are usually 150mm
DeDietrich, Baumatic are usually 125mm
Miele are usually either 125mm or 150mm, with a higher m3 t/put for 150mm
best to check with the vendors
you can get rigid round ducting in these sizes, plus the equivalent flat ducting and converters - the 5inch equivalent being 205x60mm, so it fits under a standard batten.
I have an email reply from the Elica distributor in the UK that it is usually acceptable to have a total of up to 15m of ducting for a 150mm extract with:
* 1m of rigid ducting counting as 1m
* 1m of concertina ducting counting as 2m
* each turn counting as 1m
the main problem is condensation inside the ducting so it's a good idea to run the fan on full blast every so often and NOT increase the size on the way out
another option is to have an external motor and a motorless fan - but these are expensive and the motor is usually quite large and AFAIK must have 6inch ducting
for the record we bought a DeDeitrich tube fan as we already have 5inch flat ducting fitted in the ceiling
another thing to watch is that, if you have higher-then-normal ceilings is that the fan can extend down to an appropriate height - as luck would have it this only rules out the cheaper ones!