For listing comments in your comments template in WordPress theme, you will use wp_list_comments() function. It accepts several parameters, and one of them is avatar_size. Default avatar size is 32 which may appear too small, depending on how your theme is designed. Thankfully, you can easily change this by changing avatar_size within functions’ array:
How big can Gravatar be? The simple answer is – big enough. If you check on Gravatars’ documentation, you will find that it goes up to 2048px pixels, which is quite enough. But when choosing the size, you should remember that many Gravatar images (even in smartphone era – or because of it?) are of lower quality, so anything between 100 and 200 might be, in my humble opinion, the best choice.