this Nintendo DS has two screens of resolution: 256x192

if they are put as one vertical screen this is a 256x384 screen

This is a 2:3 aspect ratio

if you consider that your viewer will use a 1920x1080 screen to see your content the largest 2:3 image that fit in is 720x1080 so using this resolution is your best option. With this resolution you control the up-scalling result and have a higher bitrate on Youtube (better image quality)