How do I use'; the blur tool'; in photoshop cs2 on a big image?

I am trying to slightly blur out some parts of a large 300 dpi image in Photoshop and the blur tool doesn't seem to work. What do I do? It works perfectly fine once I apply it to the same small low-res picture, but it does nothing when I try to use it on a big image. Any possible solutions?How do I use'; the blur tool'; in photoshop cs2 on a big image?
At the top left you will see a toolbar. This will have many options including size and how much it blurs.

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