Google Chrome is not the only application struggling with OS X Yosemite’s file dialog. When saving files in Coda, the dialog will continually fall off the bottom of the screen.
Currently the only known workaround for the issue is to switch to the simple view by clicking the up arrow next to the file name box, and then click it again to re-expand the dialog and then it will be probably sized again. However, you need to do this every time you save a file.
There is a bug in Apple’s OS X Yosemite in which the file dialog boxes grow larger than the screen. However, it does not seem Apple are in a rush to fix this.
The workaround is different for each application but for Google Chrome you can temporary solve it by running the following commands.
defaults delete com.google.Chrome NSNavPanelExpandedSizeForSaveMode defaults delete com.google.Chrome NSNavPanelExpandedSizeForOpenMode
The box will continue to grow each time you use it, so you will have to run these commands again every time the box gets too big again.