It's possible that a 23" x 40" cot sheet may be too small for a cot that measures 21 1/4" x 43 1/2". However, it really depends on how snugly you want the sheet to fit on the cot.

If you're looking for a tight fit, then a sheet that is only slightly larger than the cot itself may be too small. On the other hand, if you don't mind a looser fit, then a 23" x 40" sheet could work.

One way to determine if the sheet will fit properly is to measure the distance between the edges of the cot where the sheet will be tucked in. If this distance is greater than 23" x 40", then the sheet may be too small. However, if the distance is equal to or smaller than 23" x 40", then the sheet could potentially fit.

Ultimately, it's best to consult the manufacturer's recommendations for sheet sizes for your specific cot, or to try the sheet on the cot before making a final determination.