in INT2Ext+ thread
we can read that 100mb is required every 500mb on SD-EXT.
As once your custom rom is flashed you have more or less 100~150mb space left, a 1Gb SD-Ext is more than enought.
If your SD card is not class 10+, i would not recommend using swap partition, that could slow your phone. I prefer a good old auto-taskkill once screen is off.
Don't forget to save you contents before patitionning !