eshell-buffer-maximum-lines

eshell-buffer-maximum-lines is a variable defined in `esh-mode.el'.
Its value is

1024

Documentation:
The maximum size in lines for eshell buffers.
Eshell buffers are truncated from the top to be no greater than this
number, if the function `eshell-truncate-buffer' is on
`eshell-output-filter-functions'.

You can customize this variable.