You can never have enough space but...
Do not take on more than you can insure and heat. Again, we have stayed a credit free charity so far. We never take on more than we have money in the bank to cover. In our town there are a ton of old mill buildings. We spent 2 years doing a long survey of buildings and all the mills were under developement by the time we finished. But after long study, we determined that we did not want to be in an old mill anyway because the heat bills would of broken the bank.
Plus, most of the fabric mills were unsafe and needed major rehab. So even if we had been offered a 20,000 sq ft space it would of been a bad idea to go there if we could not heat it enough to work in the cold winter months. If you live in San Diego, perhaps this is not a problem. But even if the people could handle my cold garage the shampoo and toothpaste froze and were useless.
We worked off my 10' x 10' porch for almost 2 years by using the Internet to do precise matches of Wishes and donated goods. Holding back the public from donating everything they want to get rid of is one of the tricks to succeding at all of this. By using the Internet and our weekly e-newsletter we solicite specific donations from people that know who we are and what we do.
Then we got a network of stay at home moms in 5 towns around mine and had them use their porches as local drop spots. Once a week or so they would bring their donated goods into the central spot. Or in a crisis case, one of us could go out and collect what we needed for a client from the 5 drop spots. This system allowed us to build a very dedicated donor base pretty quickly by offering convenience. Plus, we had an ambassador in 5 towns now all telling their towns about our cause.
If you do not get into the furnitue business right off, you can work miracles out of a pretty small space with some Rubbermaid shelving and tons of time on the computer scheduling the pickups and drop offs.