I have extended in Bangkok before with a 3.5x4.5 but it was several years ago. I'd suspect you can. If you can't, there is a place there at CW you can get new ones done.
It depends on the office. Chiang Mai are hardcore about it being 4x6. I've extended in several other places with 3.5x4.5 (standard EU passport size). Chiang Mai is the only one I've been to that cares.
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