What is the max size allowed for the /dev/hda1 DOM root partition?

I replaced the 256M DOM with a 4GB CF card, so disk space wasn't an issue.
To leave enough space for add-ons a did setup /dev/hda1 with 500M.
After flashing my Premium Plus with ATV 5.3 using Morly's flashing tool (runs under Linux, still no Open Source however),
my box stopt at 49%. (It seems it stops just before starting Enigma2.)

Then I tried flashing ATV 4.2, but no go: Will boot, but fails somewhere in the startup.

At first I had no idea what the problem was. As a last resort a repartitioned /dev/hda1 to 196M.
Then ATV 5.3 started fine. My box was running again, pfff..

It seems there is some limit in mounting the rootfs a boot time. I don't quit understand..
Mounting a big 1TB hd fs for movie storage is no problem at all!

Who's the expert here?