Not sure that the function "will return the nearest genes". Flank (upstream or downstream) is generally used to return sequence.

If you use BioMart via the web and examine the source code of a page with form inputs to specify flank size, you see this:

<input type="text" 
 name="hsapiens_gene_ensembl__sequences__attributefilter.downstream_flank" 
 value="100" 
 onblur="if(filterEltHasValue(this)) {     
 checkAttributeCheckbox('hsapiens_gene_ensembl__sequences__attribute.downstream_flank');
};">

Which suggests that for the web version at least, the default value is 100.