This issue is largely solved in Stata 13, so I guess you have an older version.
You can still do so in older versions by just leaving out the equal sign, which you can see in the example below (it ran in Stata 12, in Stata 13 both macros are not truncated). This is discussed in the following article: Nicholas J. Cox (2008) "Stata tip 70: Beware the evaluating equal sign" The Stata Journal, 8(4): 586-587. It is now freely available here: http://www.stata-journal.com/article.html?article=pr0045
. // create local a with an equal sign
. local a = "`c(alpha)' `c(ALPHA)' `c(alpha)' `c(ALPHA)' `c(alpha)' `c(ALPHA)'"
. // create local b by leaving the equal sign out
. local b "`c(alpha)' `c(ALPHA)' `c(alpha)' `c(ALPHA)' `c(alpha)' `c(ALPHA)'"
.
. // local macro a gets truncated
. di `: length local a'
245
. di "`a'"
a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X
> Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T
> U V W X Y Z a b c d e f g h i j k l m n o p q r s
.
. // local macro b does not get truncated
. di `: length local b'
311
. di "`b'"
a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X
> Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T
> U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q
> R S T U V W X Y Z