You can set the auto-crop to a specific window size using Custom setting in the export dialogue, or just crop to the window-size. You can set the height and width separately, but you should set a specific font-size for the image in the main screen. If you leave it at Fit-to-width, the image will simply adjust itself to fit the screenwidth, which may not fit the height, and it will ignore spaces on either side of the text when doing this if there are no visible shapes beyond the spaces.

If the max fontsize of 288 is too small, you can highlight all the text with CTRL-A, then in the fontsize field you can type in a size factor up to 400% (min is 5%). This has the effect of making the text 4 times larger, but only in the x- and y- directions, the z direction will be unchanged, so the text depth will appear thinner. Just increase the extrusion (and the bevel) to bring the text back into the same proportions again. You can then adjust the screen window to the size you want without the image changing size on you, and crop to that.

Mike