I think t is always OK to move from a 'bad school for your child' to 'a good school for your child'. To a dgree, the size of the school is not relevant to it being a bad or good school - if you were to say anything between 100 and 600, I'd be saying 'OK'.
However, 50 - 7 kids per year group if they don't have a nursery class, probably 6 if they do - is just too small. It's easy to have a single boy / girl, or perhaps just 2, in a year group. Even by combining all the boys in years 4-6, you probably can't raise a football or cricket team. You need practically the whole school for a choir. And the finances - particularly if it is trying to operate 3 classes, and allowing any release time at all for the head - just don't really bear thinking about (though that will depend a little on whether it is in a well-funded or poorly funded county - however, as costs are generally calculated on 1 teacher per 30 children, 1 non-teaching leader per 200 or so, you can see that 3 teachers for 50 children is a LONG way from ideal). If it is a 'posh' village, they may survive with a lot of fundraising, but tbh that can feel like you are 'paying for school' because there is such a small pool of families to fundraise from.
Mixed age group classes sound OK ... but what if you are the most able in the lowest year of the mix, and are already 'top'? How will you continue to be challenged 3 years laer in the same class by the same teacher? In a larger school, you would have a peer group. In a middle sized school, with 1 year group per class, the spread of abilities would be narrower, so the teacher may well be able to differentiate for you. If you're in a tiny school with 3 years on the same classroom, the teacher is already pulled every which way - from the lowest ability child in the lowest of the 3 year groups, to the highest ability in the highest (and curricula are now much more 'year group based' than they were when I last taught 3 year groups in 1 class). Although in theory there is personalised attention, there genuinely is a limit to how far a single teacher can split their focus in any given hour of the day.
Moving from a very poor standard sized school to a good tiny school COULD still be, on balance, positive for an individual child. However, there are factors against a tiny school that just don't exist for a standard sized one, and that should be weighed in the balance.