Measuring pencils, rulers, white board markers, and other classroom objects can be okay for general practice, but I imagine it gets boring pretty quickly.
Here's a solution! Life-sized animals (listed below) for your students to measure. Each set contains 7 pages of animals that are true to size, in both standard and metric units. Straight measuring lines help kids find the correct answer. Interesting animal facts are also included for each animal. Answer keys are included.
Animals included in this pack:
scorpion, bumblebee bat, pygmy lantern shark, hummingbird, giant water beetle, Titan beetle, rhinoceros beetle, lightning bug, Atlas Moth, Atlas moth caterpillar, Giant African Land Snail, common garden snail, giant squid eyeball, Etruscan shrew, and Millie the chihauhau. Also mentioned are George, the record-setting Great Dane and a common giraffe (both not shown life size).
Eggs included in this pack:
hummingbird, chicken, cockatoo, sea turtle, bald eagle, and ostrich
*These files are very large. They may take some time to download.
*The sample files are jpegs (due to size restrictions), but the actual product is composed of PDF files.
*This may work as a whole class practice sheet, or you could use it as a center. The metric set is appropriate for 1st grade (using cm cubes), and the standard set is appropriate for 2nd-3rd grade (using cm cubes and/or rulers).
*All images are copyright and royalty-free and have been selected from Creative Commons websites.
*This product has been designed for standard letter-sized paper (8.5" x 11"), but it will correctly print on A4 paper, as well.