Thank you for your post.
The buffer size that you require depends on how long you wish to record for and at what sampling speed. For example, the ADC-212/3 has a 32k buffer. If it is running at it's maximum of 3MS/s and using one channel then it can record for a total of 10.67ms. At this point it then needs to download the data and thus there is a slight gap in sampling whilst it does this.
I'm not sure what you mean by an upper useful size... you may need to exaplin more what you mean there.
Hope this helps