The vigor of the ferment will depend on the fermentables and the yeast you use. I leave some headspace for my primary fermentations in glass carboys. Generally with that headspace, any "krausen" that rises to the top falls back without being blown off. An airlock is good. This also allows for the staged addition of yeast nutrients over the first few days of fermentation. I do the primary fermentation in 6.5 gallon glass carboys, then rack to 5 gallon carboys, filling them up to the neck for the dropping clear period.
For my fruit meads (melomels) I use an "open fermenter" that consists of a 10 gallon Vollrath SS stockpot and the stockpot cover with no airlock. This way I have plenty of room for the fruit and the vigorous fermentation that often occurs with a fruit mead. Also the open fermentor allows for "punching down the cap," which is stirring the fruit that rises to the top back down into the liquid. This should be done at least once a day. When the fermentaion has subsided (but is still active) I remove the fruit with a SS strainer and transfer the mead into a glass carboy. At this point I try to fill the carboy all the way up to the neck to avoid oxidation over the time it takes for the mead to finish fermenting and drop clear.
It's easier than it sounds once you get the hang of it. No worries.