There is no general rule for the size of Segmented Memory, other than it will be at least the memory size during normal use.
There are Siglent DSOs that have more segmented memory, e.g. the SDS1004X-E, which has 2 x 14 Mpts in normal acquisition mode but can store up to 2 x 54 Mpts in segments.
Likewise, the SDS5000X, which has normally 2 x 250 Mpts and can store up to 2 x 433.5 Mpts in segments.
The SDS6000 has 1 x 500 Mpts during normal use, but can have up to 1762 Mpts in segments.
The SDS2000X plus/HD series is limited to 2 x 200 Mpts in any case.
Of course everyone wants as much segmented memory as one can get, yet it’s also nice to have long record lengths when needed.
EDIT: Ah, and don't forget: for the SDS2000X plus/HD, segmented capture mode and any additional storage are standard, whereas with some other manufacturers they are expensive options.