When rendering to file, click the ... button to change the file name and format. If it isn't letting you enter a new name but only click on an existing file to overwrite it then something is seriously wrong somewhere.

The save as button is for saving scenes and presets, not images, and will be active only when viewing a DAZ|Studio content folder (since it isn't possible to save Poser format files).

Active viewport is limited to the size of the viewport, regardless of the size selected in render settings - to get a larger image you need to render to new window or file. To get a good qulaity render you need to be using 3Delight, not OpenGL (I know you've been doing this at least some of the time, just a reminder).

The default V3 is of course limited by not having a texture, so there's not a lot of point in rendering large other than to show there isn't a general problem with doing so - the next thing to try is applying a texture and se if you can still render to that size.

One possible source of flatness in your renders when loaded into Photoshop is the colour settings - if PS is set to assign a limited gamut colour space to untagged images (which your render will be) and then convert to a wide gamut space (if you're working in Adobe RGB for example) then the colour range of the imported image will be heavily clipped. A bad monitor profile could also cause the image to look bad in PS as it makes unhelpful corrections to the display.