One thing I see in your screenshot. In the Export PNG dialog, the DPI should be 96.0. I don't think that will fix the problem though.
After some testing, it looks like no matter what kind of bounding box you use, the Export PNG feature still thinks the image is larger than the bounding box (because of the filter). I think you will need to use one of Lazur's suggestions.
After testing with clipping, I'm not sure if it's going to work. But here's a way that I know will work. (Note that this is one of Lazur's suggestions.)
1 -- File menu > Document Properties > Page tab > Custom Size
2 -- Set the page size for the size you want the image (64 px)
3 -- Center your image on the page
a - Object menu > Align and Distribute
b - for Relative to: dropdown menu, choose Page
c - click both of these buttons
and
4 -- File menu > Export PNG, and use the Page option (will export only what's inside the page border) (be sure to set the DPI for 96.0)
I didn't look at the SVG file which Lazur attached for you, but it might be showing you how to do this too. I'm sure it's showing you at least one way to make this work. Be sure to have a look at it.