I modified the default PowerUI Manager to accept a design resolution, so you can design for one resolution that better fits all.

GUIManager.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using PowerUI;

/// <summary>
/// This is the default PowerUIManager with a much needed design resolution.
/// </summary>
public class GUIManager : MonoBehaviour {
	
	/// <summary>A File containing the html/css/nitro of your UI.</summary>
	public TextAsset HtmlFile;
	/// We like 200,600 though!
	public Vector2 DesignResolution = new Vector2 (800, 480);

	// OnEnable is called when the game starts, or when the manager script component is enabled.
	void OnEnable () {
		int DPI = Mathf.RoundToInt (Screen.dpi);
		int X = Mathf.RoundToInt (DesignResolution.x);
		int Y = Mathf.RoundToInt (DesignResolution.y);
		// Optional. This allows PowerUI to easily work with screens of varying resolution.
		UI.Resolution=new ResolutionInfo(new DesignSize(X,Y,DPI));
		// Load the UI from the above HtmlFile. Note that UI's don't have to be loaded like this! You
		// can also just set a string of text if needed.
		if(HtmlFile==null){
			Debug.Log("Please provide a HTML file for your UI.");
		}else{
			// Write the html:
			UI.Html=HtmlFile.text;
		}
	}
	
	// OnDisable is called when the manager script component is disabled. You don't need this.
	void OnDisable () {
		UI.Destroy();
	}
}