Well with the good advise and insight of others, I ended up purchasing full queen size sheets (fitted, flat and pillow cases). I went with 100% cotton and wash it out of the box with the water setting on very hot and then through the dryer. I wonder if the sheets had been prewashed, but the fitted sheet covered the 58 x 78 mattress just fine. Thanks again for everyone's thoughts and help.
During this project I installed the Froli Star System for under mattress support, replacing a hypervent. Also, a new trifold 4" thick mattress by Inofia (2-1/2" high density foam topped with 1-1/2" memory foam) was installed. The trifold serves two purposes, ease of accessing the bottom storage bins and making up the bed with clean sheets. If interested, I have written up the installation experience of the Froli Star System on the forum, search Froli to find.