Measure, Measurement
If you want to say how big a cross is, you have to measure it. You can't say the size of something without measuring it first.
Imagine trying to use your hands to show the size of this cross, but the other person shakes their head and says, "Use your words!" Ugh, okay, you'll have to measure it then.
けい
はか
None
You need to measure this cross but all you have is a cake (けい). Good enough! I'm sure all cakes are exactly the same size, so they're a reliable unit of measure.
"This cross measures two cakes wide and three cakes long," you say.