The largest plausible state of Israel would probably be
- The former Palestine Mandate
- Transjordan
- The Golan Heights
- The Sinai Peninsula
In a scenario where WW2 and the Shoah don't occur, the massive Revisionist Zionist movement in Poland would have a stronger influence on the Zionist movements, and there would be millions more potential zionist settlers, depending on conditions in Europe and migration restrictions on Soviet Jewry.
The Revisionist Zionists claimed both Palestine and Transjordan as integral areas of a future Jewish homeland based on a historical claim that the 12 tribes of Israel inhabited both sides of the Jordan river. A larger number of potential Jews making Aliyah could provide the demographic support to fulfill this ideological claim. OTL, the Revisionist Zionists abandoned their claim to Transjordan in the '50s in recognition of the demographic and geopolitical constraints of their new country.
I listed the Golan Heights and the Sinai because these territories have a small enough population and a large enough strategic importance that it would be plausible for an Israeli government to try to hold onto them long term. Golan Heights, an important water source and strategic position overlooking Damascus, only has around 100,000 people nowadays. The Sinai Peninsula in 1967 had less than 100,000 people, mostly Berber nomads.
There would be major diplomatic consequences, but it seems plausible that Israel could consolidate these territories.