sandbag recommendation - how heavy do I need?

[haring]

I have a Flashpoint Rovelight 600 monolight. I would like to take it to the beach for engagement photos and wedding photos.
The only problem is that it is a little bit top heavy. The battery is in the head unit itself.
I would like to use the light on a manfrotto light stand and a medium size softbox.

I am just wondering what size of sandbag do I need? What is your recommendation? We often shoot engagement sessions ans wedding portrait sessions on the beach. Wind is an issue.

[PiroStitch]

Is it possible for you to have an assistant hold down the lightstand? That's a better guarantee than using a sandbag to weigh the stand down. The other plus is you're not going to be lugging around the 10lb bag just to weigh down the stand.