Hi Alex,
Tricky one to explain......
A longer head for the same rod will weigh more than a short head, first decide where and how it will be used. The heads I use for Spey Casting on SH rods weigh about 15.5g to 16.5g for a 6wt and 16.5g to 17.5g for a 7wt, I make them from DT 10 or 11 lines and cut them at about 26' then fish with a 7' poly leader or about a 12' heavy mono tapered leader. The reason for the length is that I am using in confined spaces where D loop space is limited and for Speycasting a ratio of about 4 times the length of the rod is deemed to be manageable for most casters, works for me. So the head and tip (polyleader) are about 32' for use on a 9'6" rod, this is shorter than 4x the length as I am often using big flies, fast sink tips and not needing much distance. To get the weight right I am using about 32' of DT, some comes off the front to ensure the head will turn over the tip & fly and some comes off the back to get the weight & length. A head for Speycasting will be heavier than one for overhead casting too, only the upper part of the > loop is really contributing to rod loading, in an overhead cast the whole head is straight behind the rod and the full mass of the head is available for the forward cast.
A shorter head will fly for a shorter time than a longer head, less head to unroll so the casting loop turns over faster, longer head more line to unfurl, goes further but more space is needed. For a general guide, I would use the weights of the Rio Max II heads listed on the Rio website and go up two sizes, so use the weight for the ST8 for a 6wt head etc.
As for running line, don't go too thin or the head will dump.
Hope that helps.
Cheers
Ben