I was a primary TA / helper for over twenty years.
I had ten years in a two-class entry infant school, so that was sixty children in each year group. Then, when I retired I continued voluntary help in a small school, similar to your description; Reception, Yr1 and Yr2 were all in one class. Despite what one might imagine, this did work well, and gave flexibility to accommodate a wide range of abilities.
The smaller school was more like a 'family', where everyone knew everyone else, and older children could befriend or support younger ones, while the younger ones could see older pupils achieving more advanced tasks and activities, that they could aspire to in due course.
Being happy and feeling secure in the early years of schooling is important, and provided the basics of literacy, numeracy, science, ICT, the humanities, and sports, drama and music are taught and supported in a satisfactory way, then a small school environment can produce successful, confident children.