12" round sounds good depending on the height. These are 12" square and 9'-0" high. Without seeing the whole house it's impossible to tell the number of columns. You will want a center space, so 26 feet wide you will probably want four columns, but you can also do double columns if it helps the proportion. You want the space between the columns to be a vertical rectangle. So if the columns are 9'-0" high then you want no more then 8'-0" clear between.

Thank you. This is a big help.

I too thank you. Pleasing design.

Not sure if you answered this already, but do you know the exterior dimensions of this home? really just looking for the length of the front porch and the depth of the main portion of the home shown in this picture.