Why do effects grey out when canvas size increases?

Im trying to create a large image to be digitally pirnted onto a paddleboard by a factory in china. 

The desired effect is a halftone effect. The gradient i am working from, created in illustrator is taken into photoshop. The only issue is we need it to be at a high resolution to be printed. So making the canvas double the size it needs to be at 18ftx6.5ft leads the option for the colur halftone effect to be grayed out? Has anyone come across this issue before or got a work around?

Any smaller and the rasterised effect dosnt yeild a clean enough half tone when printing on large scale.