I'm about to do my first all grain brew( biab ) I'm only trying small 1 gallon batches . My question is would I be able to put it in to ferment in a 25 l8tr bucket or would I be best with a 10ltr or 15ltr bucket?
If you have a 10l bucket then use it. Otherwise, I'd agree with Clint - the fermentation will create enough CO2 to easily fill & clear the headspace on a 25l bucket.