I've recently purchased another bass with the intention of down tuning it to play some heavier stuff. I actually wanted to down tune as far as A so I have ADCF, but even when just downtuning 1 step I found that the strings were too loose. Could anyone suggest string thicknesses (gauges?) that would be useful for this purpose, as I have no idea about it at all.

Thanks in advance for any help.
Well, you're gonna want a string thicker than a B string is usually, so a .140 for the lowest.

Probably a .110 for the D

Work up from there.
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i'd reccomend getting a five string set, and leaving out the g string if you're going to tune that low, since the low one is a b. even then, you'd need a heavier set, and probably would need to get your nut recut to accomidate the thicker strings, or it'll sound like ass. and that's a job for a pro. really though, if you don't have a 35 inch scale neck, you're going to sound like wet fart with an a most companies can't even get a b to sound clean on a 34.
Well, a medium set of 5 string bass strings are 125-50, so, try 125-65 and see if that works for you.
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I second getting a 5 string set - heavy gauge.
At least 0.135 - preferably 0.140 for the low.