I am TRYING (with great difficulty) to print business cards

In Pages Preferences/Settings : Rulers panel, you can choose Centimeters. On the Pages business card template, you have short horizontal and vertical guides that indicate the individual card boundaries. Insert a Text box and expand it to fit within those guides and then on the Arrange panel, you will see Size shown as 8.88 cm width by 5.14 cm for height of an individual card on a Letter sheet of cards.

However, Pages is a royal PITA for business card creation, with template elements, or blank card template. Since time and productivity are still important, I suggest that you do one of two things:

1. Avery Design & Print online using their blank business card template.
2. Create one business card in a professional quality application like Affinity Designer 2 (free trial)
   1. Units in mm
   2. Business card template (single card)
   3. 300 DPI
   4. CMYK
   5. Press ready output (PDF)
   6. Give the PDF to a vendor like VistaPrint and let them generate professional grade cards on 16 - 18 pt Index stock so people will not fold, spindle or mutilate the thick cards. Probably 100 cards for under $50.
      1. If not VistaPrint, then your preferred print shop with offset or LinoType-**** presses.