To stay at a certain depth below the water you need your weight to equal the weight of water you displace due to your volume. Subs do this by filling ballast tanks with water to go down and eject the water with air to go up. So yes, at long as you can displace as much water as you weight you can make a sub as big as you want, assuming we're ignoring the structural challenges.
The air is irrelevant. What makes something *neutral* buoyant is that it weighs the same as the water it displaces. Many solid plastics (meaning no air at all inside) will float (positive buoyancy) because they weigh less than the water they displace. For example:
>High-density polyethylene, HDPE is known for its large strength-to-density ratio. The density of HDPE can range from 0.93 to 0.97 g/cm^3 or 970 kg/m^3.
Water is 1.0 g/cm^3. Submersibles do not vary the air inside the crew area. They have ballast tanks that exchange air and water to adjust the overall density. The [Trieste](https://en.wikipedia.org/wiki/Bathyscaphe_Trieste
) used gasoline versus water versus powdered iron to adjust its buoyancy.
>Trieste consisted of a float chamber filled with gasoline (petrol) for buoyancy, with a separate pressure sphere to hold the crew. The majority of this was a series of floats filled with 85,000 litres (22,000 US gal) of gasoline, and water ballast tanks were included at either end of the vessel, as well as releasable iron ballast in two conical hoppers along the bottom, fore and aft of the crew sphere.
What you want is the weight of the whole thing to be the same as the weight of the same volume of water. So if you make it very big, you need it to weigh more so that it won't float too much. Or if you make it small, you need it to weigh less, so it won't sink too much.
Does that answer your question?