No, they're all wrong!!
I've accumulated circular saws up to 270mm, but mostly use the smallest one which will do the job. There are some handy ones with 85mm blades even.
General purpose, though, a 165mm would do that easily and not be too much of a lump to lug about. I'm 6'3 and 16+ stone so ...
If you aren't likely to use it fairly regularly, don't get a cordless one because the battery/ies will always be flat when you want it. The cord can be a pain but you can put it over your shoulder.
I see an 85mm suggestion, but your boards won't be very flat/consistent so might wander deeper than the 27mm or so depth that'll allow.
Make sure you can look atthe thing before you buy it. Some of the cheapest ones have nasty plasticky bits which don't last. I have one where the "safety" guard stopped me seeing the line of cut so I cut a lump off it. Better ones are better designed.
They used to make 135mm ones which are fine for floorboards etc but they're out of fashion, so I'd suggest this
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if you can stretch to that. Decent make, it'll last you a lifetime and be repairable. They do refurbed cheaper ones which may be OK for the price.
Put a batten along the fence,
screwed to the fence, for beginner use. You have to have good balance and a firmly supported fence, otherise it'll be scary. It won't be the easiest, going horizontally 5/6ft up.
You must at least put a string/chalk line (or pencil but hard to see) along or just one slip and you'll make a mess.
A new TC blade won't splinter too much if you take it slowly. More teeth are better, so go slower with that 10 tooth blade.
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If you DON't have a jigsaw, then that would be more useful generally, so get a decent one with pendulum and use fine blades. Straight enough if you use a batten. If you use smooth cut blades (kerf no wider than the blade) and a batten they can still wander about.
A sharp handsaw, fine tooth, would be fine-ish about 10 tpi, if you take your time , but only if you've done some hand-sawing before. Sideways is awkward.
