You cannot store a long string in a stringin record.
However, you can store a long string in a waveform record with FTVL=CHAR. caget, medm and other clients know how to display this correctly.
Here is an example:
corvette:~>cainfo GSE-PIL1:cam1:FullFileName_RBV
GSE-PIL1:cam1:FullFileName_RBV
State: connected
Host: 164.54.160.161:5064
Access: read, write
Native data type: DBF_CHAR
Request type: DBR_CHAR
Element count: 256
corvette:~>caget -S GSE-PIL1:cam1:FullFileName_RBV
GSE-PIL1:cam1:FullFileName_RBV /cars5/Data/gpd_user/data/idc/2014/run1/eng/testing/images/testing_1/S045/stubbs_test_1002.tif
Mark