very means πολύ, αρκετά is something less. I think better fits something of quite, somewhat, rather, fairly.
Yes, we also say προβατάκι. -ακι is a general diminutif, others more rare are -ουλι, -ουδι, -ελι, maybe more, ex. χαζούλι (silly ), αρκούδι (little bear), παιδέλι (little child) etc, all neutros
I translated this as 'This sheep is fairly small' but it was marked as incorrect
I translated it as "This sheep is small enough" and it was counted wrong. (say you needed to fit a sheep into a small car to transport it elsewhere, the sheep would need to be small enough!)