You can control the size of the text using the height= option for the legend label, and the legend values. Here's a non-sense example that demonstrates how...
legend1 shape=bar(.15in,.15in) frame cshadow=gray
label=(position=left height=5pct font="arial/bold")
value=(height=3pct font="arial")
;
proc gmap data=maps.us map=maps.us;
id state;
choro state / levels=5 legend=legend1;
run;