Conciseness is very important because editors are very busy people with
stacks of queries just like yours. If it looks like it will take too long they
may just skip it. A page is probably as long as you want to make it.
If sending a query by mail, be sure to include a self addressed stamped
envelope
If you don't do this, don't expect a response either way. Very important
step. It's worth the extra postage.