Important to consider how big you are, how in shape you are, and where you'll be cutting. 36" is generally too big for what most people cut today, and was actually an uncommon size in a lot of boreal and boreal-mixed forests, Maine for example. It can also be a bit of a hindrance if you're doing anything other than felling in open areas, any sort of limbing or even blazing through tight areas is generally poor with a 36" handle.
28-30" will give you better accuracy, and may help you learn if you haven't done a lot of axe work. Can be a little more dangerous, but if you're established with a boy's axe then it may make your transition to a heavier axe easier. It's quite a jump to 3.5 pounds, 3-3.25 is a little more common when learning.
Anywhere between 28" and 32" would be good depending on your height and physical condition. It's preference and feel really. I'm a big guy, 6'2", and I prefer around 29-31", maybe even a 28". They just feel really good to work with in changing conditions and allow you to be more mobile. One of my old handles is around 28" on a 3.5 pound double-bit, I think that was pretty common. A lot of guys back then would have been smaller too. You'll only really know after some work with it, then you can try a second one a little shorter or longer.
I say straight. It should be better for accuracy while a curved handle may give some power and safety benefit. Something like the Gransfors handles is about the limit of how much curve there can reasonably be. And you can compromise between straight and curved, something like a racing axe handle or even a straight handle with a fawn's foot type of swell to retain your grip.
