The brita fill & go bottles are the same size as your average water bottle - maybe a little taller. The filter is a small cartridge that fits right inside the cover - no waiting, you just squeeze the water through the filter into your mouth!
In addition to saving money over the grocery stop method -( our bottles get used all the time, and have just come back from their 3rd Disney trip - just get new filters every couple of months of daily use.) - you don't have to carry so much into the parks for your day. In the summer - it is really important to stay hydrated. I hate to bring even a fanny pack into the park with me. I put my Brita bottle on a water bottle strap - put my park pass, etc. and a disposable poncho into my Passporter Pouch(it's like a wallet on a string) , and off I go - no bag checks!!
I tend to fill it with ice at the resort in the am - then top it off with water. At the parks - I get a cup of ice from any of the vendors - no charge! - and then top it off with water from the fountains if I need to. In the summer - that ice melts so fast - I don't need to use the fountain water!!
So - the cost per trip for me is about $2, the cost of a new filter. Water bottles in the parks are $2.50 a piece!! Definitely worth the savings!