Hi Sandra,
Though there is no required dress code for travellers to follow in Uzbekistan, although conservative dress is common throughout the country. Whenever tourists visit functioning monuments (e.g. Madrassahs), women should cover their shoulders, though in other areas women are free to wear shorts, sleeveless tops and short dresses. So really it's what you would feel comfortable in.
Please note that you may be required to take your shoes off in some places, so some comfy shoes that are easy to take off and put back on is essential.
Kindest Regards,
Monty