The maximum size of the crossword depends on the amount of space available. You don't want the clues in tiny print. For example, a 15 by 15 crossword (15 squares wide and 15 squares high) will fit on half a page of most magazines. A "weekend size" crossword is usually 21 by 21 or larger.
Easy, medium, or challenging? You'll know what your readers can handle. Don't underestimate them, though. Crossword fans have a large vocabulary and considerable knowledge. The level of difficulty is determined by the clues. Most words can have several possible clues. The word "cut," for example, can be clued as Slice, or Hack, or Stab. It can be Filmmaker's "Stop!" It can be Salon service or Piece of beef. It can be "... the most unkindest ___ of all" (Julius Caesar).
Since there is no Canadian software package to make crosswords (as of this writing) any Canadian content has to be added by the crossword constructor herself or himself. This is done by compiling a list of Canadian people and places, then keying these words one by one into the wordlists of the software, as I have done. Second, I often make Canadian clues for words that are not specifically Canadian. For example:
In the U.S.A., crosswords must never to refer to diseases or bodily functions. This convention was established by an early crossword editor. I don't feel bound by this taboo, which I find a bit ridiculous. The words AIDS, cancer, and epilepsy have all appeared in my crosswords. Foreign words are also taboo in American crosswords, but I often include some common French words. My self-imposed taboos include derogatory words (e.g. fatso, hag, hick, sissy), brand names, and religious references. I'm also careful never to write a clue that might incite regional antagonisms among Canadians, or reinforce prejudices or stereotypes.
Making crosswords with pencil and paper is an enjoyable pastime for some, but it is very time-consuming. There is good software available to make crosswords. I use Crossword Compiler. In a comparison of two crosswords, one generated by software and one made the old way, most people could not tell the difference.
Rates vary widely. My own rates are determined by the fact that crosswords are my livelihood. The New York Times pays several hundred dollars for a Sunday puzzle. You probably get what you pay for.
Each crossword constructor has his or her own distinctive mindset, attitudes and values. The constructor's general knowledge is also a factor, and a wide general knowledge will make for a more interesting set of clues. Making crosswords is a hobby for some, while for others it is a full-time pursuit. If you want to compare several crossword constructors, give them all the same puzzle solution and ask them to create the clues for the words in the grid. Then you can compare their work. The fun of a crossword is in the clues, after all.
People prefer my crosswords, according to my fan mail. I make lively, engaging clues. I delete boring words from the software. I favour the concrete rather than the abstract. I try to evoke a picture or a familiar scene in the mind of the puzzler. I try to include a line from a song. I inject humour. I avoid references to American pop culture if there's a good Canadian alternative. "Brad" in my clues is never "Pitt" -- it's "Small nail." One fan wrote that my crosswords "almost smell Canadian." A high compliment!
While there's no single word in English for "crossword constructor," the French have the word "cruciverbiste."