The most important thing to consider when choosing a Japanese tea set is the type of tea that you plan to drink with it. If you plan to drink green tea such as gyokucha or sencha, choose a small pot such as a "kyusu" or "shibori-dashi" that that will give you more control over the brew. However, If you plan to drink roasted or herbal teas that can be steeped and enjoyed for a longer time, choose a larger tea pot such as a "dobin" or a western-style tea pot.

As far as the design goes, it's really a matter of personal preference. If having a matching tea pot and tea cups is a priority for you, check out the Japanese Tea Sets link above. However, Japanese aesthetics far from demand that each piece of a tea set have the same design or even size. Feel free to mix and match tea cup and tea pots to create the perfect set for you. 

Although black tea has grown in popularity in modern Japan, green tea was the type of tea traditionally enjoyed by most Japanese people. However, if green tea is steeped for too long, the tannins released into the water turn the drink overly bitter.
 For this reason, Japanese tea pots are made to be small enough that the entire contents of the pot can be emptied into the tea cups at once.
This is why Japanese tea sets often come with a tea pot that is smaller than one that black tea drinkers might be accustomed to.

In Japan, formal tea ceremonies feature matcha served in tea bowls. However, every day tea drinkers enjoy a variety of teas served in more casual tea sets. For this reason, Japanese tea sets tend to be simple, with many only consisting of a tea pot and tea cups. More elaborate sets may also include saucers and a tea canister.