A good friend of mine started the North London LETS many years ago, and it had 500 members and was a fairly ideal size. Many people were actually living to a large degree from the LETS, without needing cash for their living needs. That person started the Fourth Corner Exchange in Bellingham, which is around that size now, but I don't think it's got the diversity of services that would be needed to make it a true "localized economy".
Regarding the second question that began this
thread, the whole idea of a mutual credit exchange is that it's relationship-based. Money is a trust agreement. When the trust is a split barter system (different than a straight barter, since the transactors don't have to have what each other wants, but can use credits to bridge needs), there is a need for relationships to build that trust. They can be enhanced by a transparent accounting system, where anyone can see what other's trades have been (online is a convenient way to do that).
I met with Tom Greco (local currency guru) when he was on a book tour stop in Seattle. His strategy (described in his latest book) is to create a business-based local currency, similar to the currency that was created in Switzerland during the 1930's between Swiss businessmen - the WIR bank. It's a mutual credit system that enabled trade even during the Depression when cash was scarce, and has survived ever since then as a complement to the trading that occurs with the Swiss Franc (Euro) system.
Anyway, Greco envisions starting with bioregional businesses who have goods and services to trade with one another as the base of the trading system, which can then add individuals into the system who also have goods/services to trade.
I like that hybrid system better than a "citizen-based" (LETS style) trading system, since these systems tend to be without major goods and services, but serve more the "candle/incense/massage" realm. I started a local currency system in Port Townsend that is still going (has about 150 traders), but what it really needs is real food and goods to make it more attractive and viable as an "alternative money system". Even if a farmer wanted to
sell produce for local currency, what would she/he do with the local currrency, if there weren't sufficient goods/services available to make it worth not taking cash for the food produce? That's the real challenge: to create a true localized economy, and then the local currency would be able to have a "current of flow" back and forth and be of real value.