You have limited options in the 59avi itself.
Set this in Initial Settings / Video Output for everything except HDMI -- but set it in Initial Settings / Options / HDMI Settings for HDMI output (the two sets of settings are independent of one another).
If your TV is a 16:9 (widescreen) TV, your choices are "Wide" mode which passes 16:9 content unchanged and stretches 4:3 content left and right to fill the 16:9 frame (distorting the image -- i.e., circles look like wide ovals), or "Compressed" mode which also passes 16:9 content unchanged but puts black pillar boxes on either side of 4:3 content so as to preserve the original undistorted shape of the 4:3 movie nestled between the two pillar boxes on either side padding it out to 16:9 width.
If your TV is a 4:3 (conventionally shaped) TV, your choices are "Letter Box" mode which passes 4:3 content through unchanged and generates black letter box bars above and below 16:9 content so as to pad it out to the squarer 4:3 shape, or "Pan & Scan" mode which also passes 4:3 content through unchanged but chops the sides off of 16:9 content so that it fills the 4:3 frame without distortion but with material cropped off either side.
In either case, the 16:9 or 4:3 content detection happens automatically once you make your choice -- even scene by scene which sometimes happens when viewing "extras" content on a DVD.
For folks with 16:9 TVs, I recommend you only use "Compressed" mode at 720p or 1080i resolution, since at 480i or 480p resolution the generated pillar box bars for 4:3 content will detract from the resolution, particularly the color resolution of the 4:3 content nestled in the middle. The alternative for 480i or 480p is to use Wide mode and use a stretch/zoom mode on your TV to compress the image back to 4:3 shape and generate the pillar box bars -- which your TV can do without loss of resolution.
If you are watching "wider than wide screen" movies, even on a 16:9 TV, you will see Letter Box bars top and bottom because they are in the content coming off the DVD (which includes no wider than 16:9 frames). The 59avi has no stretch/zoom modes to distort or crop such wider than wide screen movies to fill your TV's frame. Some TVs may offer this option, but it is unusual to find it on TVs when using high res digital video inputs, since the TV makers have not yet determined the marketing need to pay for sufficiently fast scalers to do the job.
If you prefer to watch distorted or cropped movies in such case instead of the letter box bars that are there to preserve the original aspect ratio of the "wider than wide screen" movie, your workaround would be to switch to lower output resolution, and quite possibly to analog video cables, so as to get to the stretch/zoom modes in your TV that are only available for that sort of input signal.
Note also that "wide screen" DVDs come in two flavors. The "anamorphically enhanced" or "enhanced for 16:9 TVs" wide screen DVDs actually represent 16:9 frames and are your best choice. But there are also "letterboxed" wide screen DVDs which are actually recordings of wide screen movies intended to be watched on 4:3 TVs. As such, they include letterbox bars top and bottom to pad the movie to the 4:3 shape, and all those pixels are just wasted resolution. In addition, the 59avi will detect such movies as 4:3 content (even though they include a wide screen movie), and so in "16:9 Compressed" mode the 59avi will generate black pillar box bars on either side of the movie. Since the movie already has its own letter box bars included in the content on the DVD, the net effect is you will see a properly shaped movie but completely surrounded by black on all 4 sides.