Qǐng gěi wǒ yì bēi dà bēi de bīng xiāng cǎo ná tiě, xièxiè.
（you can say it with or without 的/de/, it won't be any different tbh lol )
Qǐng gěi wǒ yì bēi dà bēi de luòlí bīng shā jiā bōbà (zhēnzhū), xièxiè.
請給我一杯大杯的波霸酪梨冰沙 / 珍珠酪梨冰沙，謝謝。
Qǐng gěi wǒ yì bēi dà bēi de bōbà luòlí bīngshā/ zhēnzhū luòlí bīngshā
Avocado in Mandarin we'd say 酪梨luòlí. I did some search on google and assumed that you were referring to some kind of smoothie made with avocado and the boba.
As for the Boba, in Mandarin the big ones are 波霸, which is pronounced as bōbà and the small ones are 珍珠, which is pronounced as zhēnzhū. However they're all Tapioca. It's just the size of them that you want to order in the drinks.
Thought you'd like to learn some more about the usage of some words in Starbucks :)
Short (小杯)：Xiǎo bēi
Tall (中杯)：Zhōng bēi
Grande (大杯)：Dà bēi
Venti (特大杯)：Tè dà bēi
Smoothie can be interpreted as 冰沙 and 奶昔, sometimes even 果泥 if it's made of fruits and it's vey thick ( not like the consistency of juice )
While 奶昔 could be interpreted as smoothie, we also interpret it from the word Milkshake so as you can see 奶昔 can be referred to two English vocabularies and they all are something blended lol
The fun of learning languages huh!! [emoji][emoji]
I hope these could be of help and can get you a drink at Starbucks haha lol