Do I need to worry about hand size when buying an ergonomic mouse?

Yes. Retailer listings and community threads repeatedly note that the Logitech Lift is tuned for small-to-medium hands while the larger MX Vertical fits bigger hands, and r/Trackballs commenters with large hands flagged the lower-profile M575S as a less comfortable fit. Checking the manufacturer's stated hand-size guidance before buying is the single most common piece of advice in the reviews.