You can take shampoo on a plane in containers of 3.4 ounces (100 milliliters) or less in your carry-on bag. All your liquid toiletries, including shampoo, must fit in one quart-size clear plastic bag, known as the 3-1-1 rule. This plastic bag must be easily accessible for inspection at security checkpoints. For checked baggage, there are generally no size restrictions on shampoo, so you can bring full-size bottles. However, it is recommended to pack them in sealed bags to avoid leaks. To summarize:

• Carry-on bag: Shampoo containers up to 3.4 oz (100 ml), all fitting in one quart-size clear bag.
• Checked bag: No specific size limit, but pack carefully to avoid spills.

These rules apply in both domestic and international travel with similar standards worldwide.