1

s/c: declare struct! [d [integer!] e [float!]] it should be

2

And just below:

12 + 4 (integer) + 8 (float) = 24 in my book, not 20 bytes

3

The returned value of DECLARE STRUCT! is the memory address of the newly created struct! value.

It is never stated that structure is static (only allocated once), so is dangerous to use inside functions (see red/red@069f335 commit)