sandbag recommendation - how heavy do I need?

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.

At the beach I would use a C-Stand not a rickety manfrotto, bags depends on the wind, they make sandbags that are empty and you can FILL with sand when you're there, that saves a ton on weight when carrying the gear in.

There's a bunch of brands, these are like the ones I use. Just bring a few empty ones.


http://www.bhphotovideo.com/c/product/816990-REG

Thanks so much!