I cant find the dimensions for the three boards required for the Y-axis assembly.
Any help.

See the Mendel materials preparation page, Thick sheet section.

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Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

You can order Mendelthickparts-reprap laser cut from Ponoko.com.
The product is from me and the design file is FREE.
I used 5mm MDF.

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Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse