This is a limitation of the edit control on the configuration window. Have you tried hitting the "..." button? That opens a larger window. However if you have already done that, all you need to do is move the alias processing into the script file.
Make a script file in the scripting configuration tab (if you haven't got one already), and select that as the script file. Then make an alias:
function my_great_alias (name, line, wildcards)
--- paste the alias stuff here
end -- function
So basically cut the alias from your alias window and paste into the script file above. Script files have no size limitation, assuming you use a reasonable editor (I use Crimson Editor, which is free and easy to use).
Then put the name (my_great_alias or whatever) in the Script box in your alias, so this function will be called when the alias matches.
There are some slight differences in behaviour from the inlined alias to the one in the script file. Principally wildcards change from %1 etc. to wildcards [1] and so on.
ie. Instead of:
Make it:
I assume your big alias is a script. If not, and you are just sending a huge amount of text, you can make a multi-line Send command, eg.
function my_great_alias (name, line, wildcards)
Send [[
.... your 50 K of text here ....
]]
end -- function