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The bigger a modifier, the "softer" the light it produces at a given light-to-subject distance.
Softness doesn't mean low contrast, although that can sometimes also be an effect, but rather to the character of the transition line between light and shadow.
But if you're using a single Speedlite, putting it in a giant softbox may result in the light being too weak, so a compromise in in order.
Your idea of a 30" box is a good one for a single Speedlite, and there's actually a specific product that I think is ideal for you. It's the Westcott Apollo 28" Speedlight kit.
Unlike most soft boxes, which are designed for use with bare-bulb strobes, the Apollo -- which can also work with strobes -- has a solid, reflective back, and the Speedlite mounts inside the box and fires backward. This retro-firing design allows maximum spread of the light inside the box for very even coverage, with or without the diffusion panel in place.
Here's a link to one source:
They also make a 50" Mega Apollo, but I think it's better used with at least two Speedlites.
I've been using the 28" Apollo as either a key light or a fill light on most of my portraits for the last several years, and I love the design, the easy setup of the umbrella-style frame, and the quality of light it puts on my subjects.
Shown below is a shot of it on on of my Manfrotto Combi Boom stands in my studio. Normally, if mounted on a straight stand, the amount of tilt available is a bit limited, but by mounting it sideways on a boom I get 360 degrees of tilt.