Question about ducting vs fan size

I currently have 5 inch ducting and fans, the only bit that is different is the carbon filter which has a step-down ring from 6" to 5". I am looking to upgrade my exhaust fan, and one of the best options I've seen is a wooden acoustic box fan. Trouble is, these start at 8". What, if any, are the problems associated with such a setup?

Eg:

6" carbon filter -> 5" duct -> 8" box fan -> 5" duct

I would rather not have to blast a bigger hole in my chimney breast or fork out for more ducting if there are no issues with this plan. What do the experts say?

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I don't really see an issue, other than smaller ducting will result in higher flow velocity which may make it noisier and will probably make your fan less efficient. Might add a little more strain on your fan too, but it should still work.

Edit: I forgot to mention to make sure you have more than enough air flow (cfm) based on the volume of your tent. You probably already know about that though, but plan conservatively to account for head loss and inefficiencies in your system.

How many cfm's is the fan u plan on purchasing. Typically keeping velocities under 750 is the rule for noise but thats for hvac. 6" will max out at around 150 cfm at 750 fpm. Above that and you will start to get air noise. I have a 180 cfm fan/filter and use 6" with no real noise issues

Get a reducer I have 8" fan but switch out my bulky 8" insulation for plain 6" so I use a 8" to 6" reducer.