Seems a bit late for pugets in the wild to be giving birth, (July or early August is more typical) but I suppose it's possible.

Read the care sheets carefully. You want to avoid stress as much as possible at this time. That means making her comfortable (again, see the care sheets on how to do that) and letting her be. No handling or disturbing. In time, she just might have some live babies and settle in enough that she'll want to eat. They generally do not eat in the late stages of pregnancy so I wouldn't worry about feeding her at all right now. Give her a couple of weeks to have those babies and/or settle in. The babies won't really require any special care (we can get into baby raising tips later) just make sure they have easy access to water because they dehydrate easily. Separate any live babies from mom so she won't accidentally hurt them.

The reason you think that she's much bigger than most snakes you see around there is that you're probably seeing mostly northwestern garter snakes, which are a much smaller species. One of the smallest in fact.

Pugets are T. sirtalis subspecies, and sirtalis' as adults can be 2.5 ft to over 3.5 ft. Northwesterns are usually around 18-24 inches as adults.

Good luck with her and do come back and keep us up to date.