It depends on the purpose of your mailing. Are you branding or selling? They're not the same thing. If you want people to recognize your name (branding) so that you can do follow up contacts (like phone or door to door) then the piece should be about your logo and service offerings, as big as you can make it.
If you are selling, then you need to focus on the service that is topical at the time of the mailing (for instance, bug controls or irrigation repairs during the summertime vs mowing and season long programs in the late winter/early spring) and create a reason to buy right now, like a limited time special, a coupon, etc. and keep it on task and focused on the specific action you want them to take (and tell them what that action is.."call us right now" "order today!" etc.)
Pieces that fail to generate are usually those that don't know what their purpose is because neither do the potential customers who see them.
As a side note, the time of week for delivery and the volume that is typical in your area can be critical to your piece being seen. Go out with the grocery mailers,valpak, etc. and your piece is buried trash. Talk to the company who is handling the mailing and see what your options are. Oversized, professionally designed piece that stick up out of the regular mail often have a better chance of getting some eyeball time than small postcard stuff. Quantity may be the game, but the winners are the one with conversion percentages. If you have to send out 10,000 little ones to get one client and I need to only send out 100 larger ones to get 3, who is saving money and getting the best return? Talk to your mailers.