I haven't seen any result size issues (and I've used it to run some commands with huge amounts of output), most of the time when I've run into problems with the result data being truncated, it has been due to my prompt() regexp being a little too simplistic, and it is matching something in the output. Be especially careful of prompts that contain dollar signs, $tn->prompt("foo$ ") will match anything that contains 'foo'...

What I usually end up doing when I need to parse output that may contain data that looks like the prompt, is to do something like this instead of using cmd:

my $tn = Net::Telnet->new();
# setup your connection here...
my $tag = "___END_OF_COMMAND___".time()."_".$$;
$tn->print("some command goes here ; echo $tag");
my($pre,$match) = $tn->waitfor( String => $tag );
my @output = split(/\r?\n/,$match);
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Another option is to adjust the prompt on the remote server to be something you can readily identify that isn't likely to appear in your command output...

my $prompt = "___NET_TELNET_PROMPT_DETECTOR___".time();
$tn->cmd("export PS1=$prompt");
$tn->prompt("$prompt");
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$t->print("ps -elf");
@ps = $t->waitfor('/your_stuff/');
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Which, if you actually read the source is exactly what cmd() does:

    ## Send command and wait for the prompt.
    $self->put($cmd . $ors)
    and ($lines, $last_prompt) = $self->waitfor($prompt);
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So no, there is no direct limit on the number of lines returned, only the indirect limit placed by the timeout.

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The intelligent reader will judge for himself. Without examining the facts fully and fairly, there is no way of knowing whether vox populi is really vox dei, or merely vox asinorum. — Cyrus H. Gordon

You might consider using the dump_log method and adjusting the timeout parameter ... that might help you or at least help to identify the problem.