I just tried it with those instructions and had it work; are you sure you followed them exactly, including quitting the application before editing? I made both the minimum and maximum size larger:
Code:
SearchField = {
paletteLabel = "Search Field";
label = "Search";
nullMenu = "(Search)";
minSize = (150, 23.0 );
maxSize = ( 300, 23.0 );
with the following result:
