Creating slider with smaller arrows requires some customization for the background image sprites and the CSS file.
Attached is ValidSlider.zip
showing a slider with custom (smaller) arrows. This is the way to resize it. There is no <img /> that you could resize, because for the arrows was used background image set through the CSS.
If you like to have several Sliders with several different sized arrows, you should create several custom skins each one with customized sprites and CSS.
You could find useful the following help article: Creating RadSlider Custom Skin
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