Shri denotes, among many other things, prosperity, splendour, beauty , abundance.
In Mantra Vidya adding a bindu to Shri makes the Rama (not Lord RAma) or the Lakshmi beejam. Which is the beejam for abundance, wealth and also prosperity.
Shri or Shriman or Shrimad is often added before a name to show respect. Shrimad denotes one who is ( or which is ) endowed with Shri.
There is a tradition of adding Shri or Shrimad before the names of Holy Texts , Holy places and also before the names of Gurus and Acharyas.
Here is what Lord Shiva orders in the KulArnava Tantram :
SriGurum KulashAstrAni PujyasthAnAni YAni cha || BhaktyA Sripurvakam
Devi Pranamya Parikirtayeth||
Hey Devi, Guru, Kula Shastras and Pujyasthana ( holy places or
places that are worthy of worship ), while addressing any of these
three the word "Sri" should be added at the start and they should be
addressed only after prostrating before them with devotion.
KulArnava Tantram, Chapter 11, Verse 43.
That is why we say Sri XYZ Purana, or Sri Bhagavat Gita and not just Gita. Similarly we should say Sri Kashi Kshetra instead of simply saying Kashi.
For Acharyas and Gurus like Sri Adi Shankara too we should also follow the same.
Take for example the Sukra Chaturvimsati Nama Stotram found in the Skanda Purana . It ends like this :
||iti śrīskāndapurāṇē śrī śukracaturviṃśatināmastōtraṃ
So, Shri is added before both the Purana's and Sukra's names. It just shows respect . And, Shrimad is just a variant of Shri with both the words being used for exactly the same purpose.