If you buy a 27" model with a 2560x1440 resolution you get about twice a much pixels as you have now. It would fit the 16:9 factor (same as 1920x1080), so movies would use the whole area of the screen. Also, those are not as expensive as the 30" 2560x1600 models.
Maybe the next best thing would be two 1920x1200 (16:10 aspect) 24" models. They are not ideal for watching movies (black bars on bottom and top), but I prefer two of them instead of two 1920x1080 ones.
If you could still find your exact model, just buy a cheap used one ant put it beneath the one you’ve already got.
Don’t mix different sizes/resolutions, as it will drive you crazy when placing the windows inside Cubase (mouse jumps up or down as soon as you pass the boarder between the monitors.
It all depends from different things like the distance between your eyes and the monitor, the angle you get with your speakers placed at the left and right side of the monitor(s), the importance of the “right” aspect ratio for movies, etc.
I started with two 20" 1600x1200 monitors (not very common these days) and a good angle for the monitors. But the picture was quite fine (small pixels), so I wished I’d taken two 21" 1600x1200 models (not very much bigger, but just the small improvement that’s better for the eyes. Your actual model has got about the pixel size of those. When 16:9 became an important factor (in conjunction with the HDMI connection and HDCP), I switched to two 23" 1920x1080 models. There I had to do the same thing as you will need to do: widen the the shelf to get enough place for two screens and two speakers. 23" was too tiny again (pixel-wise), and there was always missing something at the botton and the top. So I got the “final” setup with two 24" 1920x1200 monitors. To me, that’s the best solution, but I don’t use them to watch movies anymore.
You see, there’s quite a lot to consider, and maybe it’s like finding “the one” guitar. (Which is your favourite guitar? – Always the one I haven’t got yet …)
There is an easy way to compare “monitor real estate”. Just use a drawing software to create squares that correspond to the different resolutions (like 1680x1050, 1920x1080, 2560x1600, …). 1680x1050 pixels would be 168x105 millimeters. And if you need the naked figures to compare: 1920x1080 gives you 2.073.600 pixels, for example.
Don’t confuse inches with pixels. 1920x1080 is always the same, no matter if the monitor is a 23" or a 40" model. Inches only are important if the monitor is placed too far away from your keyboard/mouse (mixing desk inbetween, e.g.).
Sorry for being so talkative – it’s just some thoughts to keep you away from my own (quite expensive) “learning by doing” process