...
junk
junk

Model="C"
junk
junk
Size="81"
junk

Model="Default"   <---- Start of "Default" block
junk
...
junk
Size="47"         <---- Required value of size (first "Size=" line after "Default")
junk
junk

Model="A"
junk
Size="22"
junk
junk

Model="T"
junk
junk
Size="32"
junk
...

sed -ne '/="Default"/,/Size=/p' TestData.txt | sed -ne 's|Size="([1-9][0-9]*)"|1|p'