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You can define bouning box for Mesh Export in two places:
1) in Rendering Engine dock there is Limits (box)  groupbox where you can define bounding box. To see what fits to the box you can render fractal in main window.
2) in Mesh Export dialog you can use Custom Limits where you can define bounding box
Remark: In Mandelbulber 2.11 there was wrong output if bounding box had different size in each dimension. It's already fixed in 2.12 (which will be released very soon)