What is the unit number (apartment I presume) on the 11th floor or does she occupy the whole floor?
I would use the present tense and say: Her address is No. xx, 11th Floor, Block A, Rex Building, 20, George Street.
Her address is Unit A, 11th floor, Rex Building, 20 George Street. No prepositions or articles are required .
You can, if you wish say She lives in/at Unit A on the 11th floor of (the) Rex Building at 20 George Street.
Although there is apparently only one city named "Seattle", it's conventional for US place names to include the name of the state - Washington, in this case. Making this a habit can avoid such little embarrassments as arriving in Portland, Oregon when you were invited to Portland, Maine, which is over 4,000km east.It depends how precise you need to be.
' ... at 20 George Street, Seattle, Washington, USA.'
'... at 20 George Street, Seattle, in the USA.'