Some common objects that you frequently carry with you have standardized sizes, and are thus useful as an ad-hoc ruler:

things frequently in your wallet, pocket, or purse

credit card 3⅜ × 2⅛", 30 thou thick 85.60 × 53.98 mm
US dollar bills 6.14 × 2.61" 156 × 66.3 mm
US quarter 0.955" diameter, 68.9 thou thick 24.26 mm diameter, 1.75 mm thick
US nickel 0.835" diameter, 76.8 thou thick 21.2 mm diameter, 1.95 mm thick
US penny 0.750" diameter, 61.0 thou thick 1.55 mm thick
US dime 0.705" diameter, 53.1 thou thick 1.35 mm thick
Kwikset classic
(5pin)
inter-pin distance: 0.15" (C2C), 0.75" total (C2C) ...
blade width of an uncut key: 0.335" ...

things frequently in your backpack or workplace

letter paper 8½ × 11" 215.9 × 279.4 mm
AC plug (NEMA 1) distance between blades: 0.5" ID, 0.62" OD
blade thickness: 60 thou
blade widths: 1/4" and 5/16"
3-prong AC (NEMA 5) ground plug: 3/16" diameter
USB connector (male, type A) 0.6181 × 0.2952" 15.7 × 7.5mm