“Small” refers to the type of headphones the adaptor can be used with: those with partial gimbal.
The DIY option contains all the components you need: adaptor, connectors, screws. Tools, skills are all your responsibility.
Please choose carefully from the 4 options. There are 2 main variables: where your delivery address is (in or outside the USA) and if you want to install it yourself (DIY) or you want us to install the adaptor for you.