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For my refractors I tend to use a 42mm field diameter for 2" eyepieces and 27mm for 1.25" eyepieces. Even if you used 15mm or 20mm you probably wouldn't notice the slight light drop-off at the edges for visual low power. Most Newtonians are designed with a minimum 10-15mm (fully illuminated) field diameter for visual use.
With a refractor there is no reason not to fully illuminate the whole field of your lowest power eyepiece but in practice you won't notice the difference if it is a bit smaller than that.
The Berfield baffle method is fine if you want to minimise the number of baffles. You can also have no baffles and just use flocking like Tele Vue do with their refractors.
My home made scopes are all made of thin ply so I used the baffles as frames to stiffen the tube and put one every 200mm or so.
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