Would 245s fit?
 

I have a set of winter tires/rims from my last car.
225s/17s and 245s/17

Do you guys think they will fit on the FRS/BRZ?

offset?

people have fit 235 or 9 inch wide tires(however you want to call it) on with very little modification there's a thread on it, but 235 front im not sure

Yes they will fit, as long as your wheels are wide enough and are of proper offset to clear both inside and outer clearances and spokes.

However since you mention it to be SNOW tires, you need to keep in mind that the wider/taller the tires is, the larger the contact patch. The larger this is, the less tendencies for the tire to be able to "bite" the snow. Unlike summer tires which rely mostly on level surface, and more contact patch or "area" to gain traction, snow and dirt tires rely more on "pressure."

Therefore fitting overly wide tires on a very light car like the FRS can lead to less traction/braking and lateral grip on ice and snow, especially on FR cars, where bulk of the weight is not on the driven (powered) set of tires.

So my suggestion is to save those for a track day, and go burn them up while you practice drifting :) and get the right set for snow later.
So you get some practice at driving a FR car like the FRS with fairly aggressive Torsen limited slip.
It's a bit tricky, even with VSC on, as its not inherently ideal for snow for a car setup like this.

Less chance to slide and hit something with your brand new car that way.

^ umm... no.

For the same psi the contact patch will remain the same size (for the most part), not the same shape but the same area. With winter/snow tires you want a rubber compound that will be "sticky" at the operating temp ranges you are likley to experience. The other thing you want to look for is "biting edges" However that is a bit of a misnomer as the edges don't actually bite into the surface, what they do is allow space for water to escape to. What happens on ice is that there is a small amount of water on the surface of the ice, Ice due to its surface texture (on the micro level not macro) does not redily have a place for the water to go to, so instead of the tire pusing the water out of the way the tire will ride up on it. Also due to the heat within the tire as well as the frictional heat of the tire interacting with the ice, as the tire rolls over the ice it will melt again putting a small layer of water between the tire and the surface of the ice. The "biting edges" create voids and spaces within the tire which allows a space for the water to go to. On some level the amount of "biting edges" also increase the mechanical interlock between the surface and the tire as it allows the tire to better conform to the surface as well, but it does not allow the tire to bite into the surface. To do that you either need a tire with grit in the rubber, or studs.