So here is what works for my user case:

-1,5 GB: It is the easiest way to have the device ultimately become unresponsive. I remember assigning full 8GB swaps to a dedicated SD card to yield the same effect about 2 years ago. I wished this to be the best performance scenario $$ SD space in exchange for better performance but without deeper tweaking it isn't for me.

Probably it is so that the more programs are invited to concurrently writing larger chunks of data to this swap, the slower the device is getting?

-800 MB, ok a little less MB -> remains pretty responsive (see video)

-512 MB (pre cheap RAM Linux rule of thumb swap size) also works well: so far I can see no disadvantage compared to the larger stock size swap.

Video:
https://vimeo.com/109855151

on the background the device is receiving a 3GB MP3 from a computer over BT, while playing an internet radio stream to a BT speaker while all at the same time recording this stream to a WAV file on the SD card's first data partition.

This should simulate a realistic load scenario for the SD card that concurrently serves as swap disks but also as data storage.

Feel free to comment if the performance is in line of what to expect from the N900 in this scenario.