Despite some of the other restrictions within TDCJ, they are fairly relaxed about traditional mail.
There are no restrictions on the number of pages, the number of photos, or the number of other inserts such as web-site articles, newspaper cutting etc that you can send in a single envelope.
Some units don't permit stickers on envelopes but a lot do - if you are unsure either call the mailroom and ask, or if that is not practical, try sending a note in a small envelope with a sticker on and see if it goes through or not.
Some units deny anything that looks bulky, calling it a "package".
Some units do restrict the size of paper or card that you can send. For example, Coffield have told me that the longest side of the paper should be no longer than 18 inches (so effectively, no posters).
If the mailroom do deny any items, they will issue the inmate with a denial slip that has a copy for the inmate to send to the free-world person.
You also do not have to put a return address on the traditional mail. Which is good when sending postcards!