Silpat too big, safe to use in oven still?
It might not be.Contact your ovens brand for really accurate info.
I know it would be detrimental to use that way in my ovens,all have circulating shelf/bottom designs and a complete fit would interfere.
Some manufactures/brands have warranty voids,noted in the manual due to finish damage on the oven floor.
I use my silpats in very hot ovens, I think they are made to withstand extreme temps so I would probably guess yes, it's safe.
Another thing you can do is trim your silpat with scissors. They cut quite nicely, with no risk of unraveling. I've cut a full sheet size silpat in half to fit my half sheet pans and they still work fine and are in good shape.