Hola, le he echado un ojo al código y tienes una etiqueta <label></label> mal cerrada...
tu lo tienes asi:
<form name="form1" method="post" action="buscador_grandeza.php">
<label>Buscar por Colonia:
<input type="text" name="txtBusqueda" id="txtBusqueda"><br>
Seleccione Municipio*:
<select name="selCiudad" id="selCiudad">
<option value="-1" selected>Ver todas las Propiedades</option>
<option value="39">Abasolo</option>
<option value="40">Agualeguas</option>
<option value="41">Allende</option>
<option value="42">Anáhuac</option>
<option value="43">Apodaca</option>
<option value="44">Aramberri</option>
<option value="45">Bustamante</option>
<option value="46">Cadereyta Jiménez</option>
<option value="47">Cerralvo</option>
<option value="48">China</option>
<option value="49">Ciénega de Flores</option>
<option value="50">Doctor Arroyo</option>
<option value="51">Doctor Coss</option>
<option value="52">Doctor González</option>
<option value="53">El Carmen</option>
<option value="54">Galeana</option>
<option value="55">García</option>
<option value="56">General Bravo</option>
<option value="57">General Escobedo</option>
<option value="58">General Terán</option>
<option value="59">General Treviño</option>
<option value="60">General Zaragoza</option>
<option value="61">General Zuazua</option>
<option value="62">Guadalupe</option>
<option value="63">Hidalgo</option>
<option value="64">Higueras</option>
<option value="65">Hualahuises</option>
<option value="66">Iturbide</option>
<option value="67">Juárez</option>
<option value="68">Lampazos de Naranjo</option>
<option value="69">Linares</option>
<option value="70">Los Aldamas</option>
<option value="71">Los Herreras</option>
<option value="72">Los Ramones</option>
<option value="73">Marín</option>
<option value="74">Melchor Ocampo</option>
<option value="75">Mier y Noriega</option>
<option value="76">Mina</option>
<option value="77">Montemorelos</option>
<option value="78">Monterrey</option>
<option value="79">Parás</option>
<option value="80">Pesquería</option>
<option value="81">Rayones</option>
<option value="82">Sabinas Hidalgo</option>
<option value="83">Salinas Victoria</option>
<option value="84">San Nicolás de los Garza</option>
<option value="85">San Pedro Garza García</option>
<option value="86">Santa Catarina</option>
<option value="87">Santiago</option>
<option value="88">Vallecillo</option>
<option value="89">Villaldama</option>
</select>
</label>
<input type="submit" name="cmdBuscar" id="cmdBuscar" value="Buscar">
</form>
y debería estar así:
<form name="form1" method="post" action="buscador_grandeza.php">
<label>Buscar por Colonia:</label>
<input type="text" name="txtBusqueda" id="txtBusqueda"><br>
Seleccione Municipio*:
<select name="selCiudad" id="selCiudad">
<option value="-1" selected>Ver todas las Propiedades</option>
<option value="39">Abasolo</option>
<option value="40">Agualeguas</option>
<option value="41">Allende</option>
<option value="42">Anáhuac</option>
<option value="43">Apodaca</option>
<option value="44">Aramberri</option>
<option value="45">Bustamante</option>
<option value="46">Cadereyta Jiménez</option>
<option value="47">Cerralvo</option>
<option value="48">China</option>
<option value="49">Ciénega de Flores</option>
<option value="50">Doctor Arroyo</option>
<option value="51">Doctor Coss</option>
<option value="52">Doctor González</option>
<option value="53">El Carmen</option>
<option value="54">Galeana</option>
<option value="55">García</option>
<option value="56">General Bravo</option>
<option value="57">General Escobedo</option>
<option value="58">General Terán</option>
<option value="59">General Treviño</option>
<option value="60">General Zaragoza</option>
<option value="61">General Zuazua</option>
<option value="62">Guadalupe</option>
<option value="63">Hidalgo</option>
<option value="64">Higueras</option>
<option value="65">Hualahuises</option>
<option value="66">Iturbide</option>
<option value="67">Juárez</option>
<option value="68">Lampazos de Naranjo</option>
<option value="69">Linares</option>
<option value="70">Los Aldamas</option>
<option value="71">Los Herreras</option>
<option value="72">Los Ramones</option>
<option value="73">Marín</option>
<option value="74">Melchor Ocampo</option>
<option value="75">Mier y Noriega</option>
<option value="76">Mina</option>
<option value="77">Montemorelos</option>
<option value="78">Monterrey</option>
<option value="79">Parás</option>
<option value="80">Pesquería</option>
<option value="81">Rayones</option>
<option value="82">Sabinas Hidalgo</option>
<option value="83">Salinas Victoria</option>
<option value="84">San Nicolás de los Garza</option>
<option value="85">San Pedro Garza García</option>
<option value="86">Santa Catarina</option>
<option value="87">Santiago</option>
<option value="88">Vallecillo</option>
<option value="89">Villaldama</option>
</select>
<input type="submit" name="cmdBuscar" id="cmdBuscar" value="Buscar">
</form>
fijate que la etiqueta label, la cierro justo cuando termina el texto que contendrá el label... y tu estabas dejando dentro del label todo el <select...>, el <input...>, etc
Un saludo!