The replacement symbols for <space>: (%20 and +) are equivalent when encoding a URL query string. See this SO question and answer:
https://stackoverflow.com/questions/2678551/when-to-encode-space-to-plus-or-20
If the responding REST service cannot handle this, it should be fixed.
If you actually need %20, you have 2 options:
First determine parameters $MyBrand and $size and their values and then pass the parameters along in the location like this:
.