Microsoft has been slowly expanding the reach of Windows Phone with support for new display sizes and resolutions recently, and it looks like that trend is set to continue with the upcoming Windows Phone 8.1 update. Information has been pulled from the new WP8.1 SDK
that shows the platform will be adding new resolution support in both the high and low spec categories. This adds on to the info we've already gotten from the new SDK
According to data pulled from the SDK by self-described "Microsoft fanboy" Roman L, Windows Phone 8.1 will be adding support for both qHD (540x960) and QHD (1440x2560) displays. Previously the maximum resolution was 1080x1920, which was added with the GDR 3 update to Windows Phone 8. Additionally, the minimum logical resolution is being dropped from 480x800 to 360x640, and Microsoft will be implementing a responsive design scheme. This means that developers need to update apps that use absolute positioning or absolute resolutions, and create apps that can be upscaled to various resolutions.
It also implies that Windows Phone will soon be absorbing Windows RT. With the GDR 3 update, Windows Phone gained support for 6-inch displays and larger, and the 1080p resolution. Now, Windows Phone is getting support for QHD, which is a resolution best suited for tablets rather than handsets right now. There have been rumors of QHD displays on handsets, but those are still rare, so it seems more likely that Microsoft is building more towards tablets.