There is no legal minimum population for a State to have (if there were, North Dakota might have to worry about reverting back to being a territory in a few decades) but as a practical matter I can't see Congress granting Statehood to any territory that has less than half a Representative's worth of population.
Here's the current population estimate of US territories, commonwealths and associated states as of July 2004:
American Samoa: 57,902
Baker Island: 0
Guam: 166,090
Howland Island: 0
Jarvis Island: 0
Johnston Atoll: 396 (non-indigenous)
Kingman Reef: 0
Marshall Islands: 57,738
Federated States of Micronesia: 108,155
Midway Islands: 40 (non-indigenous)
Navassa Island: 0
Northern Mariana Islands: 78,252
Palau: 20,016
Palmyra Atoll: 20 (non-indigenous)
Puerto Rico: 3,897,960
Virgin Islands: 108,775
Wake Island: 200 (non-indigenous)
With the exception of Puerto Rico, none of these have enough population for Statehood to be a possibility.
Of the remainder, Johnston, Kingman, Midway, and Palmyra should really be annexed by Hawaii. (All but semi-submerged Kingman were claimed by the Kingdom of Hawaii.) Unihabited Baker, Howland, and Jarvis Islands could also be annexed by Hawaii but are remote enough and lack historic ties to Hawaii, so keeping them under the control of the Interior Department also makes sense.
A merged Mariana Islands (Guam & NMI) at 244,342 has an outside shot at becoming a State, but I don't think their economies could stand the strain at this time of no longer being exempted from some US labor and tax laws because of their terrirorial and/or commonwealth status. The Northern Marianas were the only portion of the old Trust Territory of the Pacific that opted for closer ties rather than free association status, so I wouldn't be surprised to someday see a united Mariana becoming the 51st or 52nd State in a couple of decades if their population and economy keep growing as they have been.
Given how the rest of the former Trust Territory (FSM, Marshalls, Palau) has fractured politically, I can't see them reuniting, even if it would gain them Statehood somehow, and they certainly wouldn't unite with the Marianas. Wake was never part of the Trust Territory, but the Marshall Islands is trying to claim it.
American Samoa is too small and not in a good geographic position to merge with any other US territory. A merger with Samoa if it is even politically possible would not likely lead to Statehood but rather to associated state status like that held by FSM, the Marshalls, and Palau.
As for the Virgin Islands, even a possible merger with the British Virgin Islands would only bring the population up to around 130,000 and I see no chance that any other of the Leeward Islands would even consider such an action.
3 of those places listed are independent nations:
*Palau, Marshall Islands and Micronesia
