True, Sigma cannot use all of his abilities in any of the listed form, but he is capable of chaning forms. For instance in X4, he can choose one of the three forms: Alien, Cyborg Head, Three Heads, and he can change form when he wants to.

The "Using Huge Form will result in a sure lost" is debatable. Sigma's Huge Forms still vary in size, some are just 2-3 times the height of X, while Colossus Sigma is about 30 times his size. I see what you mean, having such a huge body in a small area will certain reduce the mobility, yet there is one thing: Sigma is invulnerable in most of his body part.

Even though this is a rather small stage (the main platform is 50 Feet in diameter?), there are enough room for normal sized Sigma to move around and dodge things.

Also, being a maverick, he has the damage barrier (in some forms, not only the huge forms) just like X and Zero. For instance, Gunner Sigma, Kaiser Sigma etc.

His pet Velgauder would probably help a little, but not too much. Jumping around shooting Fire and Ice, etc.

I am going to elaborate a bit on the different forms.

Alien/Cyborg/Heads Sigma:
Switching form when he finishes an attack or two.
The Laser from Alien Sigma are HUGE, and will hurt a lot.
The Cyborg Head can suck in or blow away the enemy to a dangerous spot, the lava pit for instance.
When Sigma is in the Three Head forms, he is totally invincible. The three heads will shot Fire, Ice, Lightning, and move around pushing the enemy to the spikes or...Lava, in this stage.

X5 Sigma:
Dexterous. He can jump and air dash, create orbs that bounce around. He can also cast 3 shields which rotate away him. They will damage whoever touch them. While they can be destroy (and take the damage for Sigma), but Sigma can simply recast them.

His most feared attack would be his Energy Waves. Casting waves of energy from different height, giving the opponent very small room to dodge.

Gamma Sigma:
His Spikes on his fists can be dangerous, and he can crash his fist toward the opponent and push them off the edge. Or make them off balance (for those who fly...), putting them in a dangerous position.
He can also shoot Orbs which will home at the opponents current position two times, which will make their next move more predictable. He can also send out Electric Current from his fingers.

What's interesting about this form his, he can change from Solid to Wireframe from time to time. While in WireFrame form, he can do the attacks but he is invunlenable.

I'll elaborate on Kaiser Sigma and Colossus Sigma when I have the time.

P.S. I do not like Sigma. I put up the arguments so that this battle will be more interesting. ^_^