Of considerable size (5)
I believe the answer is:
large
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of considerable size</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for large that I've seen before include "Sizeable" , "Regal (anag)" , "Titanic" , "Big, almost regal" , "Big - 5 down (anag)" .)