I was thinking of making 'belt' or something that my wife could put on to pretend that she is hugely pregnant. The thing is that should be comfortable enough to allow sex.  I mean like  pregnant with 10 children o more.
Do you have any ideas hot to make it or seen something like this? 

Fake belly is what I would suggest

Moonbump.com

I was thinking about something bigger. Lets sa6 fantasy -size.

Pretty sure you aren't going to find anything that is fantasy size and comfortable.

Search for 6' Balloons on Amazon, eBay or other. You'll likely have to just use air to get the size you are wanting, but if you're willing to sacrifice extreme size for a more realistic size and weight, these will work really well with water. The balloons are quite thick when they aren't over expanded, though I usually double them up when using water, just to be safe.  

A nylon/spandex unitard are great to hold the balloons in place. If you want to try the water route however, you may need a couple of unitards and/or some belly bands or maternity support underwear to help hold it in place. All the extra layers might not be in favor of what you are after. You might just have to get creative.