Format size
Format size property defines size of output files.
1:1 option allows converting of the images as is. Drop down list 
(Dots per inch) gives additional quality parameter.
Max possible size is unavailable when output format is AutoCAD™ DXF.
Fit to max size means that the longest side of image will be equal to the entered number. In this case the shortest side of image will be counted in accordance to proportion of the input sizes.
Fit to min size means that the shortest side of image will be equal to the entered number. In this case the longest side of image will be counted in accordance to proportion of the input sizes.
Width and Height allows setting custom sizes of output image without keeping of proportions. If 'auto height' is checked then
Height is counted pro rata of entered Width (when selected). And vice versa.
Note:
If one of 1:1, Fit to max size, Fit to min size, Max possible size or Width and Height is chosen it automatically switches off all the others.