This is very simple. The R means radius, while the number is millimeters. So, a 1000R monitor forms a portion of a 1000mm radius, determining its curvature.
Keep in mind monitor curvature and size are not the same and the 1000mm does not refer to the width of the monitor, but only to the radius.
What you should note is that the bigger the number before the R, the less pronounced the curvature. So, a 1000R monitor is more noticeably curved than a 2000R monitor, for example.
The larger the monitor, the less likely it is to be very curved, as that requires more complex manufacturing and may be difficult for many people to get used to. Sure, there are 1000R 49” monitors, but most in that size class opt for 1800R or even a wider radius, up to 4000R.
Using this logic, if you want to experience a really curved monitor, going with 1000R and a display in the 32” to 34” range is probably the best option. However, it’s entirely up to personal preference. Many people prefer bigger displays with a light curve, and so you may want to do more research before deciding.