There is no option to limit size in spool command as below:

SQL> help spool

SPOOL
-----
Stores query results in a file, or optionally sends the file to a printer.

SPO[OL] [file_name[.ext] [CRE[ATE] | REP[LACE] | APP[END]] | OFF | OUT]

However you can use below Unix Split and mknod command to split the spool file.

cd /tmp
rm spool.txt
mknod spool.txt p
split -b2000m </tmp/spool.txt &
sqlplus / as sysdba

SQL> spool tmp/spool.txt
OR
SQL>@<any script>.sql #put the spool file path as /tmp/spool.txt

You will get multiple spool files with maxsize 2000m and name as xaa xab xac xad and so on at /tmp location.

SQL> !
[xxx]/grdbms/home> cd /tmp
[xxx]/tmp> ls -ltr x*

prw-r--r-- 1 grdbms grdbms 0 Jun 24 18:48 spool.txt
-rw-r--r-- 1 grdbms grdbms 1003249664 Jun 24 18:48 xaa