Use the fontsize property. Also, use the string property for the string:
str1 = uicontrol('unit','pixel','style','text','fontsize',20,'string','choose one scene!!',...
'position',[250 250 300 20],'units','normalized','backgroundColor',[0.4,1,0]);
More info:
web([docroot '/techdoc/ref/uicontrol_props.html'])