With JVC I wouldn't go much bigger than a 120" minimal gain white screen. No reason to be using a grey screen unless your room sucks. In which case, get something other than a JVC projector as the blacks will be lost on you.
Reading things, I have a very hard time justifying any and all 4K hoopla. There is no content at all in this format, and absolutely no need for it.
JVC has a phenomenal projector in the RS65 I'm sure - but at almost $12,000, it's not at all worth it.
jnr: Save your money. If you want a good JVC projector, then get their RS45 or RS55 for thousands less, enjoy it for a few years, then you will see when/if 4K has a real market, and you will see 4K projectors at half the price they are now.
Seriously, I see the RS45 at about $2,999 street, while the RS65 will be closer to $9,999 street. Maybe a bit less.
In anything other than perfect conditions, I would bet most people won't be able to tell them apart.