There's a character-countdown readout just above the box where you type your messages. See it? (To open a text messaging window, click the down-arrow on the Compose button, then click Text Message.)

Messages can be up to 158 characters. And yes: spaces count. ;o)

158 characters looks like this:

Hey Joel we got to the place but didn’t see you so we split and came back to Paulina’s where her and Doug have all this food - we’ll probably stay awhile. L8R