Magnetic heatbed options - can I trim these to size?
Adhesion and sizing
First of all, the product you link is very likely a BuildTak clone or a similar product. My Ender 3 came with a similar one and I have changed to a different one after I ripped it off accidentally to mount a similar one. I have yet to find a sheet of this material that you can't cut to size, as it is pretty much only a polymer with a strong tape on one side and a rough surface on the other. So trimming is not a problem at all.
Mounting
To mount your new surface, you need to get a surface onto your heatbed that has

good contact on all the area
securely holds the bed in place
is as flat as possible

So you surely want to get a metal sheet that is as flat as possible to mount your surface on. If your bed is heated, your mounting method needs to transfer the heat as good as possible, so a direct contact to the heated bed would be good. This, however, rules out most thin magnets, as these magnet sheets demagnetize under heat, limiting the usability for higher temperature materials, for example, ABS. The original Prusa uses a spring steel bed and high-temperature magnets embedded in the heating platform to work around this.