Command blocks, in singleplayer, are capable of displaying messages up to the cap of 254 characters, using /say or /tell.
On a server, these /say commands are truncated, to a limit of ~120 characters displayed. There is no issue entering the characters on the command line, it's just that only 120 characters are displayed on a server.
For screenshot context- the 1st picture is someone testing a map creation of mine, in SMP with one other player. The other picture is in singleplayer & creative, triggering the same command block manually.
1. Open up a world in singleplayer, and place a command block. Open the command line, type "/say" and as many letters or words as possible. Activate the command block with redstone. It should work as intended.
2. On a server with command blocks / permissions enabled, do the same thing. When the command block is activated, only two lines will be visible, which will only be a portion of the entered "/say" command output.