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Refractors are fun, good on you for having a go. It makes sense to start with something simple and inexpensive and learn as you go.
As Brian suggested you can draw out the light cone full size to get an idea of baffle apertures etc. If you don't have a piece of paper big enough then you can scale the drawing such that the vertical axis is 1/1 scale and the horizontal axis is 1/2 or 1/4 or 1/6 scale or whatever works for your focal length.
Here is a picture of the plan for my 5" f12 Istar. For the diameter of the focal plane I used 42mm as this is the field stop of the largest eyepiece I expected to use. If you are only using 1.25 inch eyepieces you could use a diameter of 27mm or even a bit less. Once you draw the lines from the focal plane to the lens aperture you can then derive the baffle apertures for any position along the focal length. In my case the baffles were equally spaced as they also served as frames within a plywood tube.
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