Headspace/blowoff tube

I brewed a basic wheat beer yesterday - 1.046 starting gravity. Put 5.5 gallons into 6-gallon carboy. Pitched Wyeast Am Wheat straight into it - no starter.
20 hours later and my airlock is flooded and its dripping down the sides and I've got a mess. Second time in last three batches this has happened.
Obviously, I should just start with the blowoff as standard procedure and I will.
But this has me thinking about another related issue. I want to start doing smaller batches. Something more in the 2.5- to 3-gallon range. But all I have are six-gallon buckets and carboys. And I'm told that fermenting a three-gallon batch in a six-gallon carboy can be an issue with all of the extra headspace.
Yes, I can just go buy a smaller fermenter. But the question is, where is the balance of headspace if you have to do a blowoff tube every time?

I'm not sure if I have the exact right answer for you, but I can't think of a reason why using a 6-gal carboy would be bad for a small batch.

In primary, the yeast will generate so much co2 during fermentation that oxygen would pretty much be non-existent when the primary fermentation is done. Plus, if you were to rack to a secondary I read a great head space solver where you would use sanitized glass beads (typically used with fish tanks) to fill up the extra volume of the 6 gal carboy making air almost a non issue.

Aside from that I don't think there is really a standard for headspace where you would know whether to use a blowoff tube or not. Though OG, type of beer, , yeast used, and head retention of said beer would all be factors to consider. Experience in that department would really be the governing factor all in all.

I brew 2 1/2 gallon batches in a 6 1/2 gallon bucket fermenter with no problems. As mentioned, fermentation will give off enough CO2 to easily fill that space. The problem with the extra headspace is if you secondary because the fermentation should be done so there is no CO2 being generated. Without the CO2 blanketing your beer, odd organisms can get a start if your sanitation isn't superb.

What temperature are you fermenting your beer? I keep mine in a room where the temperature is 62 to 64 degrees and have yet to need a blowoff as the ferment remains slow enough to keep the krausen low until the yeast slow down. Then I bring it to a warmer room to complete the ferment. When I ferment in a bucket whose lid doesn't leak, my airlock only shows about one bubble per second at the most.

Secondary is my bigger issue. I don't necessarily use a secondary just for the sake of using one but it seems the bulk of what I brew has a dry-hop or other need for secondary. I suppose the easy solution is just spending 20 bucks for the three-gallon carboy next time I order. Then I can primary in the bigger buckets/carboy and move to secondary in the three-gallon.

As for temps, it depends on the season. I live in the high desert and high/low temps fluctuate a lot. I ferment in a closet and right now, it's between 67 and 70. In the summer, when we're running the AC pretty steady, it'll be a tad lower.
What really had me miffed last night when I posted was the fact that I had to clean up a mess late a night. It was a wheat recipe and the same recipe that I brewed a week ago - same type of yeast and OGs were 1.046 this weekend and 1.045 last weekend. So, I didn't expect to have a mess.