Recently Seeedstudio revamped their online PCB order system. Now I have to manually type in the dimension of each board. Any number can be typed in regardless the actual board size. It used to automatically detect board sizes. I wonder if it is some artifact on my computer or not. Have you recently (within last 3 months) ordered from seeed and do you have the same issue with DIY board size? I wonder what happens if I DO put in the wrong board sizes.
The board size on the web form is only relevant for pricing information, it's not a mechanical specification. The cost of 10 (standard, 2-layer) PCBs is $9.90 USD regardless of whether your board is 20x20, 50x50, or 100x100. However, the pricing does begin to change with dimensions, below 100x100, when increasing layers or changing material (etc).
Answer to Q1: Yes - the auto dimension feature is gone
Answer to Q2: You could get an incorrect quote