The rules of the senate are simple: two seats per state. So the new state would get two seats on the senate.
The rules of congressional appointment by state are:
- There are 435 seats
- Each state gets as many seats as its approximate share of the total population of the United States
- But each state always gets at least one seat
There are currently about 322 million people in the United States, which means each state gets one seat for about 740,000 people. The proposed region of the new state of Jefferson houses 457,859 people - not enough to warrant a second seat beyond their guaranteed one.
When Alaska and Hawaii were added to the union, their two new seats were simply added for the rest of the legislative period to avoid having to kick any elected congressmen out, so it's likely that the same solution would be adopted in this case. After that, the seats would need to be recalculated based on the population numbers. Considering that the majority of inhabitants of the proposed state currently live in Oregon, it is likely that Oregon would lose one seat (from 5 down to 4). But depending on how the population numbers develop in the next years, the result could also be a loss for a different state.