You want a very low altitude, to maximize drag, to make the object deorbit quickly even if you never manage to establish control from the ground. The constraint is that you don't want to go too low, or you will not achieve your desired mission duration.
There's a handy website at https://www.spaceacademy.net.au/watch/debris/orblife.htm , based on the book Satellite Orbits in an Atmosphere - Theory and Applications (Glasgow, 1987) by Desmond King-Hele. Speaking in very general terms about a phenomenon characterized by large uncertainties, they suggest a rough estimate of orbital lifetime as one day at 200 km, one month at 300 km, and one year at 400 km.
The exact details depend on fluctuations in the sun's energy output, which changes the density profile of the upper atmosphere. The most common parameter used to quantify this effect is the 10.7 centimeter radio flux (https://www.swpc.noaa.gov/phenomena/f107-cm-radio-emissions). The other thing you need to improve the lifetime estimate is the area-to-mass ratio of your satellite. The drag force is usually calculated as proportional to the cross-sectional area presented to the flow (https://www.grc.nasa.gov/www/k-12/rocket/drageq.html), so the acceleration caused by drag is proportional to area divided by mass.
Given those two numbers, you can use the curves in the first site or the original book to make a more precise estimate of how long your particular satellite will last in a particular state of the atmosphere. Until then, however, 300 km circular sounds like a reasonable design point. That only specifies semi-major axis and eccentricity, so the other four parameters can be chosen to meet other constraints, such as the location of your ground station and the location of your launch site.