The previous inlay and all its glue will need to be removed prior to installing a new one.
Currently, the sizes listed are what are available at this time.
No special cleaning method is required. Clean your tub as you normally would.
Yes you can cut these with tin snips, industrial sheers, a Dremel tool or handsaw, then sand edges back down at angle to recreate bevel. There is a 14.5 x 32 sized kit available as well.
It is not hard to install, you just have to follow the instructions but you are going to need the help of some one with a little knowledge of construction.
You just have to find some adhesive suitable for the job, or buy a new kit.
There is a bit of flex to the Repair Inlay, but I suggest that the bottom of the tub be very nearly flat.
The tub inlays do not have a pre-cut hole for the drain. Yes, they can be trimmed with industrial shears, tin-snips, aviation shears, small hand saw, etc. Once trimmed, sand edges back down @ 45* angle to match bevel around inlay.
If your priority is sealing the cracks and having a nice finished look, this is a good product, considering the alternatives.