What are the dimensions of this item?

Specifications: * 300 watts RMS @ 0.5% THD into 4 ohm load * Signal to noise ratio: 105 dB (A-weighted) * Dimensions: 11" W x 7" H x 4" D* Cabinet cutout: 10" x 6-1/4"* Line-in and high-level in crossovers: Adjustable 50 to 150 Hz, 12 dB/octave low-pass crossover, and a fixed 330 Hz, 12 dB/octave low-pass crossover* LFE-in crossover: Fixed 330 Hz, 12 dB/octave low-pass crossover

Wuill it push an 8-ohm load?

I asked BASH directly about what it can do, and they were very responsive. It WILL push an 8-ohm load, but at half the wattage as it will at 4 ohms. It is NOT stable at 2 ohms. If in doubt, ask BASH directly.

is this rated at 300W @ 8ohms? My subwoofer is rated at 250watts at 8ohms so if this is rated for only 4ohms that's too little.

Being rated at 4 ohms, this amp required at least 4 ohms of resistance to remain stable. With that 4 ohms of signal resistance provided by a sub, it allows 300watts of power through to feed the sub. Your sub provides 8 ohms of resistance, so it is only letting around 150 watts through to the sub. Therefore, you might consider the Bash 500 instead. It is 500watts at 4 ohms, so an 8 ohm sub will allow around 250 watts of power through.

I am looking at the Acoustic Audio HD728 7.2 (two passive subs). Can this amp drive both subs, or would I need two amps?

yes, you can, but keep in mind that the nominal power for the amp is 300 W, you won't be able to extract more energy from the amp. (300 W is still a lot for a subwoofer!). You might want to connect the two subwoofers in Parallel and that will reduce the total ohms from 8 to 4. You can also connect them in series, but that is going to increase the resistance (to 16 ohms) and that will mean that the transfer of energy won't be optimal (i.e. they will sound with low volume)