The site states that the maximum resolution is 32x64, however, the example games seem to be at 128x64.
How can I achieve this?
A standard CHIP-8 interpreter's default resolution is 64 pixels wide by 32 pixels tall.
If you target Super CHIP-8 (SCHIP) or XO-CHIP, it is possible to switch the display into a "high resolution" 128x64 pixel mode. In Octo assembly language this is accomplished with the "hires" instruction. The larger examples shown on the jam page use XO-CHIP instructions.
edit: I suppose that it's also worth noting that Octo offers a feature which rotates the display in increments of 90 degrees; the equivalent of tipping the TV on its side or upside-down. You can find this in the "Options" tab of the toolbox. Some games have taken advantage of this to produce a portrait-mode display at 32x64 or 64x128.