I have tried multiple saw sizes and multiple bar sizes for milling. I am not an expert but I have a couple years experience. Ill share my thoughts
bar length
36” IMO is the biggest you want to go to keep things portable-ish. 36 fits the alaskan mill nicely if you take off the dogs on your powerhead. If you choose to keep them on, its still lots of room for the majority of logs here you will find in the west coast area.
I live on the east side of the rockies and mpst trees here are 12-24” diameter live for fir trees. Down by the river you will find larger But I only fell what i can manage.
powerhead
I cant recommend the 395xp enough. It is far superior in stock form to the 390 and even the 3120xp in stock form. Remember i said stock. You can make the 3120xp the ultimate milling saw for power but it requires quite a few mods to make it worth while.
The 395 has an excellent oiler and will keep up with the 36” bar no problem given a sharp chain. It has an outboard clutch and trust me this is what you want for milling. You are keeping alot of the heat of that clutch away fr the motor promoting longer saw life for the task of milling. Once your chain is broken in and stops stretching you will not be worried in the least about having a front chain tensioner. you will be only using one bar and one chain for a milling saw and once the chain is set it will not stretch given you sharpen it properly.
the 395 has plenty of power for 36” bar in live softwood as well, and will carry full comp very well for the best finish compared to skip on your wood planks. It doesnt require a muffler mod or anything else to promote this power either and when tuned properly runs at just the right temp never getting too hot.
the air filter, which is important for milling, on this unit is superior in every way to every other current husqvarna saw. It has a rubber boot that has an excellent seal via a clamp to the air horn of the carb setup. It will not pass particulates and has a very large surface area to draw from, almost as much as the 3120xp
particulate seperation via the cooling fan on the side of the saw also seperates large chips from small chips extremely well and even the small chips to not get into the filter that much. This is important because you dont want a coked up air filter after 5 passes on your milled work, you want to keep going.
the 390xp is a great saw, its one i do not own but the inboard clutch is designed for quick chain/bar changes and is not designed for the heat of milling. This is not to say it will not mill, i have milled with the 372 inboard clutch, but it is not an ideal setup for a long lasting power head in this type of Application. .
the weight between the 3120xp and the 395xp is extremely noticable as well. Again for a first millingg saw that will serve you well right out of the box for milling is the 395.
i have recently retired my 395 and upgraded to a 3120xp. I had to modify the air filter, the coil and the carb to get it to run properly milling. This cost alot of money and time and if I had the 3120 years ago as my first milling saw i would have been disappointed given my experience back then. Only after the mods has it finally stepped up to the level and beyond the 395 for milling.
again this is just my experience, take it as you will. Others may chime in and say that its *B-S.... and the 3120 beats the 395 any day for milling. I strongly disagree in its stock form.