To select  a large area, there are several techniques you can use.

Click on the top left cell, then use the scroll bars to expose the bottom
right cell, then shift-click on it.

If your top left cell is at or near the top left of the spreadsheet: Select
the bottom right cell. Then shift-ctrl-home. If your top left cell is not
to be A1, then use shift-right-arrow and shift-down-arrow to move the
selection to your top left cell.

If you only want a few hundred cells: click on the top left cell. Then use
shift-page-down to extend downwards and shift-right-arrow to extend to the
right.

and so on. Adding shift to almost any movement will extend a selection
area.