*{ font-size:12px; margin:0; padding:0;}
Method 1:
#testBox{border:1px solid #cccccc;padding:5px;width :220px;
min-height:100px; /*The minimum height value is set to: 100px*/
height:auto !important; /*Compatible with FF, IE7 also supports the !important tag*/
height: 100px; /*Compatible with ie6*/
Method 2:
/*This method can also be used alone*/
height:expression(this.height < 100 ? "100px" : this.height "px");