This morning in Japan, TV Osaka broadcasted a clip on the DSi LL, which features a comparison image between the newly designed portable and the other handhelds in the DS line.
The above image really gives a good idea of the size difference, while this clip displays the big screen in action.
This mock-up DSi LL apparently shows how big the LL is when opened. Big!
「ニンテンドーDSi LL」実物画像集 [オレ的ゲーム速報＠刃]