Clothing in Japan and working

So I am planning on going to Japan for a year after I graduate next summer. At the moment I am looking at applying for the JET programme and also saving money up in case I don't get on that and have to go it alone and pay for TEFL and the such. I worked out I am going to need to work while I am out there of course and all I have heard about is teaching English and not much else. So ya is there anything else I could look at whilst on a working holiday? Also, this is the tricky one. How difficult will I find it to get clothes in Japan? I am a UK size 16 at the moment, normally I run a 14 but all sorts of issues have meant I put the weight back on (But I am going to the gym and aiming for a 12 by New Year!) But even a 12/14 I know is quite big in Japan right? Length wise I will be fabulous as I am a petite just about 5', in fact a 28inch leg length is too long for me still >.< Its the er... width xD Should I take loadsa clothes with me?