Does anyone know a way to get dots that are smaller than the Maximum Radius of 4; or maybe a workaround?
If you want smaller dots as compared to the overall image size, as a workaround I suppose you could upsample the image then apply the halftone effect. I work almost exclusively on photos at upsampled resolutions myself - it just gives you so much more flexibility.
Dots with fewer pixels than that won't give a very good halftone.
Upsampling, filtering, then downsampling again may be your best bet.
Chris and Noel,
Thanks for your suggestions. That was the only workaround I could think of too.