Hi, new member here. I am designing an Arduino shield with a lot of ICs on it, with mass production in mind. It is meant for the Arduino Uno but will be a lot bigger ... nearly twice as long. Being new to Arduino shields, I'm wondering if it's common practice to make such large shields. The alternative would be to break it into two smaller shields that stack together. I'd like the Arduino community's input on how to best design it.
is it though hole or SMT
no its not really common to have oversized sheilds, though no one says you cant either
You can make your shield whatever size you want.
Larger PCBs cost more to purchase - trade that off against the need for more connectors and other needed interconnects between the 2 boards that could complicate the design.
It's mostly SMD. Connectors will be through-hole though, for better stability.
nearly twice as long...It's mostly SMD.
Must be a complex board then, what does it do (or is that a secret at this point?)
Rob
It's a video game system. 
The core stuff can fit on a smaller board, but I decided to add USB, SD card, and an on-board AVR coprocessor.
Sounds like the host Arduino will be a shield for your board 
Rob