Well, maybe somebody else can jump on here too, and give you more info...
I would say that with regards to the teat size, yes, there is some degree of trial and error involved. It often takes them a few days to get used to a faster flow. With my 6.5 mo, I went to a Size 3 avent teat at about 5 months, but that transition took me about 3 weeks. I kept switching back and forth btw the size 2 and 3, bc she too, has a lot spill from around the sides. This still happens, so I often feed her with a bib on. When she is really motivated, however, or drinking in her sleep, she hardly spills any, which makes me think it is the way she is sucking on it, not that it is too big. You may have to experiment for a while, and there is nothing wrong with staying on the lower flow teat as needed.
Food quantities are quite variable. Some babies eat little, others will keep accepting food until you need to cut them off yourself, lest they lose their appetite for the bottle. From my experience, and what I see on this board, a 5 mo usually eats only about 1-2 tbsp/oz at one time. Most of the baby good jars here are about 3.5-4 oz, which is maybe 120-150g. That is a lot of expect a 5 mo to eat at one time. My 6.5 takes about 1.5 jars of baby food spread out over the entire 3 feeds a day.
HTH, and I hope someone else can add to this.
