Who is asking: Parent
The is not a unique answer to your question. The answer depends on the dimensions of the inside of the box you build to give a volume of 1.25 cubic feet. One such box interior is diagramed below.
You could however get a box of the same volume if its interior dimensions were 1/2 foot by 2 feet by 1.25 feet.
There is also a question about the top of the box. Does it have a top?
I am going to assume that the interior of the box is as in the diagram and that it does not have a top. In ths case the interior is 12 inches by 12 inches by 15 inches. (1.25 feet is 15 inches.) The outside dimensions will then be 16.5 inches long by 13.5 inches deep by 12.75 inches high.I hope this helps,