Many of the people who tackle my puzzles would probably enjoy the additional clues a bigger grid would offer. At first sight, however, it would seem that everyone else involved: the publisher, the less sharp-eyed solvers and I, would be worse off.
The publisher's designers and typesetters would lose the symmetry of the two size-matched puzzles on the usual page (Alan Shuker's general knowledge crossword that appears alongside mine is also 13 by 13) and, worse, they would have to squeeze more clues into the same space. (Subeditors and typesetters, I know, don't like squeezing copy.)
The larger grid would have to be scaled down to fit the page, so reducing the size of the squares and their inset clue numbers. As a result the designers would lose their symmetry and the solvers would lose some visual clarity.
On top of all that, I would have to write more clues. So why do I want to change?
The 13 by 13 grid is simply too restrictive. The number of fair, well-connected 13 by 13 grids is quite small, and because I like to change the pattern each time, I am frequently stuck, given the nature of these beasts, with several three-letter answers or a lazily deficient looking number of clues.
Worse, I am now running low on three and four-letter words that I haven't clued several times before. (The English language is strangely deficient in polite and common four-letter words. There are plenty of uncommon ones but I am loathe to use those. I prefer to leave West African trees and Scottish watercourses to Scrabble nerds.)
Recurring words are the most troublesome. It becomes ever more difficult to find fresh clues for them, especially when one of the previous clues turns out to be a plum. I can rarely beat my own best performances.
A bigger grid would add variety and novelty. The main objection I can think of is that more clues might sometimes impose typesetting problems, but I have tested that lately with some puzzles carrying an exceptionally large number of small words and hence many "normal length" clues and they have been printed without complaint (at least to me).
There are ways to make a 13 by 13 grid with more words than normal. This is done by “skewing” the grid slightly. The downside of this technique is that one ends up with a large number of similar-length words, often of four or five letters. Boring.
Altogether, the using the 13 x 13 grid has become increasingly like trying to dance in a phonebox.
A larger grid would be a great liberation, giving me access to a far greater variety of words and answers. I'll have to consult the editor, but first, would anyone in my vast army of followers here mind?