What line do you mean?
A line in the files list?
An input box?
If you are not comfortable with the size of the input boxes, you may use the extended tags dialogue (Alt-T) which has individual dialogues for each input.
Otherwise, the ordinary windows behaviour for input boxes applies. How do you cope with long URLs in your Internet browser?
If you enter an input box, ususally, the whole text is selected.
Pressing a key for printable character overwrites the selected text.
Pressing a cursor key (up, left) unselects the text and moves the text cursor one character to the left, starting at the end.
Pressing a cursor key (down, right) unselects the text and moves the text cursor to the end of the text.
Pressing Home jumps to the beginning of the text
Pressing End jumps to the end of the text.
Pressing Ctrl-Cursor lets the text cursor from start of word to the next.
Press als Shift selects the text between the last and the next cursor position.
These functions should allow to jump to any spelling mistake quick enough as I doubt that spelling mistakes only occur at the beginning of a string.
Where would you like to have the change?