Most states that I have seen did different sizes depending on the number of digits in the plate. I believe the standard plate size of 6X12 for North America went into effect in 1957.
Here is a set of 1952 Illinois vehicle plates showing the different sizes from a single digit up to the seven digit plates. Not just a depression era thing.
Some of the digits on the left of the five and six digit plates are obscured, sorry about that.
-Tim