Well at this time Rome had alliances with several surrounding Latin based towns as well as Capua. However, Livy makes no mention of Rome's complete military strength. But from that you could probably make an estimate. So probably around 150,000 max strength.
As for a hypothetical defeats at Sentinum (295BC), it was a decisive battle for Rome. If Rome had lost, they would of had a hell of a time recovering. The 35,000 stong force of Umbrians, Gauls and Samnites would have crossed the Appenines and caused some serious damage. Roman confidence would be smashed while the enemy's would be elevated to an all new level, this easilly could have caused Capua and other Latin based towns in alliance with Rome to desert to the Samnites. The Romans would then be fighting badly outnumbered in a war where they could be attacked from all directions. Even before they were holding off attacks from the Etruscans/Gauls in the North and the Samnites/Umbrians in the East. Sentinum was of massive importance. It would be like if Hasdrubal had defeated Nero at he Metaurus. After Sentinum the Romans convinced the Gauls to retire and from there it was all downhill.
As for Aqualonia (293BC), well the Samnites didn't have a chance. Their confidence was virtually nought and they were fighting by threat of death from their officers. The Roman morale on the other hand was excellent and the Consul Papirius Cursor was a highly experienced and respected general. The Samnites were pushed back the entire battle and when they were led (falsely) to believe that Roman reinforcements were on the way, they all retreated, even the officers and generals. If by some miracle they had won then I think it would have only been a setback from their enevitable destruction. The Gauls and Etruscans were out of the picture by this stage and Aqualonia marked the last major stand by the Samnites. They had almost their entire force of 40,000 men there, if by some miracle it had been victorious it would of had to immediatly deal with another Roman army under the consul Spurius Carvilius who was at this time smashing another Samnite force at Cominium. They themselves knew this in the battle and it was the speculation that Spurius would appear that sent them to rout.
So Sentinum was a must-win victory but Aqualonia was a sure-thing victory.