If you have plans to ultimately print, you should start at that size and then downsample your for-the-screen versions.
At most print shops, the maximum print size is about 32 X 24 inches. At 300 dpi, that's an image resolution of 9600 X 7200. If you've got a very beefy machine, you could do that. Likely, though, at poster viewing distance, you could get by with a print resolution of 150 - 200, which significantly reduces your working resolution.
When working in Photoshop, it is very easy to reduce the size and quite difficult to increase it. So work at the biggest resolution you expect to need and downsample from there.
You should probably talk to your print shop ahead of time so you know what their requirements and capabilities are.