A Unity Editor script to list all the atlases in your project and to show their file sizes when built.

ViewAtlasSize.cs

#if UNITY_EDITOR
using UnityEngine;
using System.Collections;
using UnityEditor;
using UnityEditor.Sprites;
using System.Collections.Generic;
using System;

[ExecuteInEditMode]
public class ViewAtlasSize : EditorWindow {

	private Vector2 scrollPosition;

	[MenuItem("Window/View Atlas Sizes")]
	public static void Init() {
		ViewAtlasSize window = (ViewAtlasSize)EditorWindow.GetWindow (typeof (ViewAtlasSize));
		window.titleContent.text = "Atlas Sizes";
		window.Show();
	}
		
	public void OnGUI () {
		GUILayout.Label ("Atlas Sizes\n", EditorStyles.boldLabel);

		scrollPosition = GUILayout.BeginScrollView(scrollPosition);

		for (int i = 0; i < Packer.atlasNames.Length; i++) {
			string atlas = Packer.atlasNames[i];
			List<Texture2D[]> atlasTextures = new List<Texture2D[]>();

			atlasTextures.Add(Packer.GetTexturesForAtlas(atlas));
			//atlasTextures.Add(Packer.GetAlphaTexturesForAtlas(atlas));

			GUILayout.Label("\t" + atlas, EditorStyles.label);

			foreach (Texture2D[] textures in atlasTextures) {
				foreach (Texture2D texture in textures) {
					if (texture == null) {
						return;
					}
					GUILayout.Label("\t\t" + texture.name + "(" + texture.format.ToString() + ")" + " - " + GetMemorySize(texture) + "MB");
				}
			}
		}

		GUILayout.EndScrollView();
	}

	[MenuItem("Assets/Get File Size")]
	public static void PrintFileSize() {
		if (Selection.activeObject == null && Selection.activeObject.GetType() == typeof(Texture2D)) {
			return;
		}
		Debug.LogError(Selection.activeObject.name  + " - " + GetMemorySize((Texture2D)Selection.activeObject) + "MB");
	}

	private static float GetMemorySize(Texture2D texture) {
		return (float)Math.Round(((double)(texture.GetRawTextureData().Length) / 1000000), 2);
	}
}
#endif