In the real world a 1/4 sheet is 8-inches by 12-inches instead of the 9X13 many people think it is.
Also, a half sheet is more of a 12x16 instead of a 12x18. I have no idea why wilton made the 12x18 pan, maybe because it's the biggest you can go in an average home oven. And if you use their 11x15, well, it's small.
I switched to using fat daddios sheet pans years ago. The 8x12 fits perfect in the classic quarter sheet cake box once the sides are iced and borders at the bottom. Also, the 12x16 fits in the commercial boxes with icing and borders. AND, when you use the full sheet boxes, two 12x16s fit side by side just right, no trimming needed.
I love the fat daddio and they bake up beautifully. I have quite a few of my wilton cake pans develop pin holes in them. The metal on the bottom just flaked away and they have holes.
I have made sheet cakes daily at work for 25 years, and at home weekly as well. Back in the days, we would literally cut the sheet cakes in half, and half again and they always fit in the box perfect, I couldn't understand why I had such problems getting my home cakes to fit until I whipped out a measuring tape and compared sizes. I bought the fat daddio and didn't look back!