allow for explicit size specification

[hexaeder]

Sometimes, I need to generate temp arrays for a specific size. I think the patch is straight forward and should not break anything.
I am a bit unsure about the ReverseDiff extension, though.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.