I can tell you depending on season , Juninho wore M, L, XL and even XXL.
Sounds crazy i know , heres most of the match worn kit sizes by year home and away unless stated
95/96 All players wore XL apart from branco xxxl
96/97 Min player size is XL , Majrity are XXL
97/98 Most XL , Long sleeved all XXL 1st team
98/99 All XXL
99/00 All XXL ( But some shirts later in the season were more true to size in Home matchworn, key is the level of curve in the nameset. All matchworn away shirts are XXL and the majority were long sleeved.)
Juninho wore Medium and Large in this season (99/00)
I can go on if anyone interested, but its not until around 09/10 when players really started to specify their own size.
There are the odd anomaly with some players asking for specific sizes , but its quite rare pre 2009.
Player shirts pre 2006 are much easier to authenticate , i wont give it away, but if anyone would like any advice , please dm me here or on X formerly twitter. i will happily help any fellow boro fan .
As for the shirt in question, yes it is way way over priced , but if thats what the lad values it at then thats up to him.
As for its authenticity , well it looks very good , would need the shirt in hand to verify its real.