You can try to set the form element's parentTheme
to css_xhtml
and wrap elements on the same line with div
and apply your style afterwards.
something like
<s:form action="test" theme="simple">
<div>
<sj:a id="ajaxlink" onclick="disminuyeBloque()" button="true" buttonIcon="ui-icon-minus" cssStyle="height:20px"/>
<sj:textfield id="tamannobloque" name="tamannobloque" readonly="true" size="2" parentTheme="css_xhtml"/>
</div>
<div>
<sj:a id="ajaxlinkminus" onclick="aumentaBloque()" button="true" buttonIcon="ui-icon-plus" cssStyle="height:20px"/>
<sj:slider id="echo4" name="slider" label="Echo" value="{3, 7}" min="1" max="10" onCompleteTopics="sliderRangeStop" cssStyle="margin: 10px; width:20%" parentTheme="css_xhtml"/>
</div>
</s:form>