1) Where can i find clothes for size XXL etc.
2) Is there anyway to find spring clothing in early January?
How about XXL for ladies?
Generally, Japan is not a good place to shop for large-size clothes.
Uniqlo takes some pride in having a wide range of sizes, but some less popular sizes may be available only online or in their bigger stores. Their Ginza store in Tokyo is one of their largest, so it would be worth a try. Note that a Japanese size XXL would be equivalent to an American XL.
Sakazen has a ladies floor,
Nissen is one of popular online shops for fashion; they have large size (=大きいサイズ / Smile Land）and seems sold out quite quickly. The site unfortunately in Japanese only; and you can get your ordering at 7-11 convenience store or address in Japan.