When specifying a pixel value for your inline style prop, React automatically appends the string "px" for you after your number value, so this works:
var divStyle = {height: 10}; // rendered as "height:10px"
ReactDOM.render(<div style={divStyle}>Hello World!</div>, mountNode);
See Inline Styles for more info.
Sometimes you do want to keep the CSS properties unitless. Here's a list of properties that won't get the automatic "px" suffix:
animationIterationCountboxFlexboxFlexGroupboxOrdinalGroupcolumnCountfillOpacityflexflexGrowflexPositiveflexShrinkflexNegativeflexOrderfontWeightlineClamplineHeightopacityorderorphansstopOpacitystrokeDashoffsetstrokeOpacitystrokeWidthtabSizewidowszIndexzoom