Bags
Measure the widest and tallest points of the largest object you carry. Compare them with the opening and interior dimensions. The outside width can be wider than the usable opening.
A measuring checklist for online shopping
Sizing and care notes from Luxury Atelier.
Shoes
Stand on paper in the socks you plan to wear and mark your heel and longest toe. Measure both feet. Compare the larger measurement with the size chart and check whether it lists foot length or insole length.
Belts
Measure a belt that fits from the buckle pin to the hole you use. Compare that number with the working range of the new belt. Total strap length includes the material beyond the holes.