I would go for the 36in mill. The mill has rails at the top which bolt to cross-pieces down to the bar. The maximum distance you can set the cross-pieces apart is determined by the length of the top rails and the length of the bar, and determine the widest piece of timber you can mill by what will pass between them.
With a 2ft bar, assuming it's a roller-nose one, the furthest out you can go is about 5in from the end to avoid pinching the sprocket. If you don't take the dogs off the powerhead, you lose the bottom couple of inches. The clamps themselves use about a further inch of bar each. This gives you maximum width of around 15ins. Allowing for lumps and bumps on the wood the widest log you will get through without any prior work is a little over a foot. Of course, that's only to split it at its widest point. If you come up a bit, shave off a side and then roll it over to do the other side, you can make sure it's a nice, uniform piece of timber and goes through easily. This is obviously a bit more work in rolling things around, but allows you to comfortably process up to about 20in logs.
You also need to consider your powerhead. In my limited experience, on modern high revving saws 72cc.s will cope happily with a 15in milling cut, an 066M (93cc.s) copes with around a 2ft cut and for anything bigger you really want a 100cc+ saw. You can push these numbers a bit depending on species, but if you go too underpowered you will find it painfully slow and wonder why anyone does it!
Based on the above, a 36in Alaskan with a 36in bar and 066M
makes a good combination (max cut 27ins). A 42in bar would go nicely with an 076 and make maximum use of the 36in Alaskan since you lose the nose area.
Oh and do buy a ripping chain, and remember (literally, time) your first cut and note the width to get a sq.ins/min cut rate. Then, when it feels slow, you can measure the time and width again and get a sense of whether the chain is dull - it makes far more difference than cross cutting.
Rather a long answer, hope it helps!
Alec