First, where any test strip is placed is far more important than its size, as Drew mentioned. If you can't a good representation of the image's values in one strip, might as well use a whole sheet.
Enlarging to 16x20 was my standard (4x5 negs) for a long time. Usually a third of a sheet for the test strip, a second if for some reason I could not pull enough info off the first one. Then a full sheet test print at the selected exposure (& contrast), sometimes called a work print, to study in order to judge what exposure, burning/dodging, and/or contrast change will be needed to attempt the first print. I had to be careful not to give up too easily on an image at this point, as there was still a lot of magic to work on it before it matched what I saw/felt.
I can see being concerned about 'wasting' paper by using a full sheet if the paper itself was rare and hard to find.
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