what's the china fir wood?.

no, it is not a kind of fir wood though it called china fir wood, its genus name is Cunninghamia honours, for its details, pls see the blow description in Wiki.

/wiki/Cunninghamia

can the design or size be cusomized?

yes, we can customized it as your requirement

can you stain it for me? which kind of paint you normally use

yes, we can, we use the water-based paint normally

how long it will keep well in outdoor?

it can keep well in outdoor environment for more than 5 years even without any protect, china fir wood has great ability in outdoor, pls don't worry.