First of all, maximum power isn't always the best choice. In fact, if you want best quality of coverage, it can prove to be harmful. After all, even if your access point is screaming at the top of its power, it doesn't mean it can hear the quiet client trying to reply to it. Worse, it can result in more interference being locally generated (all radios produce some, and it's relative to their output power), it can result in other network devices also having to raise their own power, and it can result in more clients that are out of range trying to connect and thereby using up a lot of your air time. That's not even counting the social convention of potentially causing interference to neighbors.
2.4GHz
Frequency wise, there are only 3 real, independent channels in the USA: 1, 6, and 11. Every other channel will have some overlap with these three. If you configure an access point to use one of these channels at 20MHz bandwidth (please don't use 40MHz on 2.4GHz. It will only hurt you unless you have no nearby neighbors!), that is a good start.
Next, it's worth doing a WiFi survey of your area to see what is or is not in use. OpenWRT can do this for you, or you can use a tool like NetStumbler, LinSSID, or uncountable others. Sadly, most of the 2.4GHz band is in heavy use unless you're well isolated from your neighbors. If you have a lot of neighbors with a lot of competition on the band, you might want to bump up the coverage cell density (Advanced Settings). This will allow your AP to remove support for the lower speeds and become more optimized for faster clients. However, setting it too high can result in older or slower devices not being able to connect.
Once everything is able to connect, you'll want to lower the maximum output power until you still have coverage where you need it but the power is at or close to the lowest setting that will provide that coverage. This way, your AP isn't working so hard and it's less likely to cause harmful interference or get responses from devices that are truly out of your intended range. Although many people ignore this step, it's technically required by law in some places as well as being beneficial for reliability of your AP and connections. It also helps that doing so makes your network more friendly to others by allowing them to also reduce their output power and by doing so, it eliminates a screaming match between network devices.
5GHz
With the 5GHz band, you might have more freedom. However, everything has trade-offs. If all of your devices support 802.11ac or 802.11ax, you'll probably get better results using one of the middle range channels. However, these are DFS channels and are therefore subject to being shut off if your AP detects a radar using the band. However, devices supporting only 802.11n may or may not be able to connect depending on which version of the spec their radios follow.
Having your radio set to 'auto' for the 5GHz channel allows it to automatically switch as necessary to avoid interference or DFS shutdowns, but that comes at the down side of having 1-2 minutes of down time each time the AP has to switch frequencies or get connected.
As for the bandwidth setting, the trade-off is speed vs. quality. The wider you set it, the more trouble you'll have with interference, range, and connection quality. Therefore, it's best to start narrower and work your way up only if you need to do so. Personally, I keep my 5GHz radio set to 40MHz and it's still fast enough to meet or exceed my internet connection.
As always, once everything else is settled, it's time to reduce maximum output power until you reach barely above the minimum acceptable quality over your coverage area.