Go easy on gimmicks. Give your reader solid information. Beginners should start small. Expand as your profits and experience grow. You can eliminate paper waste, if you are standard in your mailings.
Standard sizes go for envelopes too.
Odd shaped envelopes may be attention-getters, but they have to be custom made, at very high cost. The advertiser's answer to a letter should take into account the express wishes of the inquirer. Use the YOU approach in your advertising copy. Most people read direct mail newsletters. They keep people up to date and interested.