If the visual condition is good (eg. on nights when the sky is clear and stable), the top usable magnification for a 60mm (2.4 inch objective lens) scope will be around 2x60=120x. This is more than enough to see the rings of Saturn, cloud belts on Jupiter and many star clusters and nebulae. So, 60mm scopes have been the standard size for a wide range of applications. Two types of these models are available at nipon-scope.com, one is Nipon 15-46x60 Spotting Scope, the other is Nipon 700x60 Telescope.
With a 70-80mm scope, however, the amount of light it gathers is about 55-65% more than a 60mm scope. This improves image quality especially when viewing at high powers and under low light. A selection of these models includes Nipon 20-60x70 spotting scope, 70/300mm refractor scope, 70/350mm refractor scope, and 26-78x78 compact zoom scope. In general, larger scopes outperform smaller ones, but they also tend to be bigger and heavier, and more expensive.