Limit WWW.texture file size?

Here's an example modified from Unity's WWW.texture docs:

 using UnityEngine;
 using System.Collections;

 public class Example : MonoBehaviour {

     public string url = "http://carillon-beach.com/images/plates/large/beach-1.jpg";
     public int maxResolutionX = 512;
     public int maxResolutionY = 512;
     
     IEnumerator Start() {
         // load the texture
         WWW www = new WWW(url);
         yield return www;
         Texture2D texture = www.texture;
         Debug.Log(string.Format("{0}, {1}", texture.width, texture.height));
         // resize the texture to fit in the desired bounds
         if (texture.width > maxResolutionX || texture.height > maxResolutionY) {
             float widthRatio = (float)maxResolutionX / texture.width;
             float heightRatio = (float)maxResolutionY / texture.height;
             if (widthRatio < heightRatio) {
                 texture = Resize(texture, (int)(texture.width * widthRatio), (int)(texture.height * widthRatio));
             } else {
                 texture = Resize(texture, (int)(texture.width * heightRatio), (int)(texture.height * heightRatio));
             }
         }
         Debug.Log(string.Format("{0}, {1}", texture.width, texture.height));
         // apply the texture to the renderer
         renderer.material.mainTexture = texture;
     }

     // returns a texture resized to the given dimensions
     public Texture2D Resize(Texture2D source, int width, int height) {
         // initialize render texture with target width and height
         RenderTexture rt = new RenderTexture(width, height, 32);
         // set as the active render texture
         RenderTexture.active = rt;
         // render source texture to the render texture
         GL.PushMatrix();
         GL.LoadPixelMatrix(0, width, height, 0);
         Graphics.DrawTexture(new Rect(0, 0, width, height), source);
         GL.PopMatrix();
         // initialize destination texture with target width and height
         Texture2D dest = new Texture2D(width, height);
         // copy render texture to the destination texture
         dest.ReadPixels(new Rect(0, 0, width, height), 0, 0);
         dest.Apply();
         // resume rendering to the main window
         RenderTexture.active = null;
         return dest;
     }
 }

When run, this example should log 900, 507, the original dimensions of the image, and 512, 288, the dimensions of the scaled image.

It's using RenderTextures, which requires Unity Pro. If that's an issue, your best bet would be to check the width and height of the texture that's loaded, and if it's out of bounds choose a default texture instead and discard what they loaded. Example:

 using UnityEngine;
 using System.Collections;

 public class Example : MonoBehaviour {

     public string url = "http://carillon-beach.com/images/plates/large/beach-1.jpg";
     public int maxResolutionX;
     public int maxResolutionY;
     public Texture2D defaultTexture;
     
     IEnumerator Start() {
         // load the texture
         WWW www = new WWW(url);
         yield return www;
         // set material to default texture if texture is out of bounds
         if (www.texture.width > maxResolutionX || www.texture.height > maxResolutionY) {
             renderer.material.mainTexture = defaultTexture;
         } else {
             renderer.material.mainTexture = www.texture;
         }
     }
 }