I'd personally go 6x8. Most economical building materials come in 4' or 8' dimensions, so if you plan around that you can reduce cutting and waste. A 6' isn't bad as you can half a sheet of plywood to get a 2' wide strip. 2' cutoffs can be used for nests and such, at least the frame. You get a larger coop for about the same cost. In Maine you might be better off with a bit more room in the coop this winter. When you come up with your design share it with us so we can critique it. That might save you some problems later.
The biggest safety problem with a tractor is that they set on the ground and it is really hard to stop digging predators and still retain the ability to move them around. Most of the time, the ground is not level so there are gaps beneath bottom of the tractor and the ground. Certain critters like raccoons, skunks, and possum don't need much of a gap to just push through, let alone need to dig. Lots of people use tractors and don't have these problems but there is a risk.
I don't know how old those chicks will be by the time you get the coop built. Life often gets in the way of building coops, it often takes you a lot longer to finish than you expect. It can take an effort to build a tractor too. It sounds like you are going to be integrating the chicks with at least two adults too, that takes more room. A separate shelter can come in really handy for integrating or build one in the coop. in the coop it doesn't have to be predator proof. Those chicks won't need a coop with roosts and nests to start with, they just need a shelter. I don't know how much of a constraint money is.
My first thought is to use the time and materials you were going to use to build a tractor and instead get that coop built before your weather turns. I don't know what your constraints are to delay that, but if you can then decide on a design and get to it. You can add roosts and nests later if that helps you get it finished. The hardest part of getting it finished is often getting started. Either leave your older chickens where they are now for a while or build a shelter inside that coop to house your chicks while you integrate.
I don't know of any specific tractor design that is any safer than others. You can look at a hoop coop design, those are often fairly lightweight so you can move them around. You can build a box out of 2x4's or even 2x2's to save weight if you are going to actually move it. Cover one end with plywood or some paneling to give them shelter from the weather and cover the other end with hardware cloth to save some weight and let them have lots of light. No nests, roosts, or pop door needed, just a simple box with the part on the ground open. 24" tall is plenty. The door is probably the hardest part to build. Access inside would be horrible but it's only temporary. The more complicated you make it the better building the coop looks.
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