So, look at you. You have almost all the questions answered. You're well on your way. I wasn't really expecting you to answer them on the forum but that's Ok as others will get good info from it.
Cutting holes in the walls... maybe that isn't necessary. You can run ducting thru windows via a wooden insert. And I have seen good systems that run ducting along the floor and out thru a doorway. I know one woman who suspended her hood from 4 posts at the corners of her bench and ran the ducts in/out thru two doors. Worked great.
Hood size. Hood size will be partially dertermined by your bench size. I like to make them so the front edge of the hood is set back from the bench edge about 6 to 8 inches so I don't bang my head into it. And I like the bottom edge about forehead high or a bit higher so my head clears it if I bend over while standing/sitting at the bench. In addition to hood size you will need to calculate fan size. The fan CFM is determined by the hood opening in square feet times 125. ie. 2' x 3' hood + 6 sqrft x 125 = 750 cfm fan size. In reality something between 600 and 750 will work.
It sounds like you want to just put a fan in a window in front of you and call it good. it may work but will be very very marginally effective. Try to come uo with a hood - fan setup that works well.
Glasscraft used to sell a hood package but it had some problems with inadequate fan size. If you search back for a pic of one you will see what looks very much like a water tub for watering stock. Something like that works well, is already made, and can be easily modified to suit.
All these things you have considered and answered should have planted a seed of understanding and a basic plan of what you need to do.
You're getting there.
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