I have several objects approximately 35ft. long and all are within 10%
of each other in length. Can I be correct in saying that the objects
are "substantially" the same length?

The Oxford English Dictionary says that the adverb, "substantially,"
means "in essentials, to all intents and purposes, in the main."

Much of the definition depends on the situation.  Objects within 10%
of the size are very close -- unless you know in advance that length
will be a key criteria.

By counter example, if you had a loan for $35,000 and repaid $32,000
of it they would probably argue with your definition that you'd repaid
"substantially all of it."

Google search strategy:
use of New Shorter Oxford English Dictionary

For a look at how senstive the word can be in legal definitions, the
following search strategy gives many examples:
definition + substantially

Best regards,

Omnivorous-GA

I'm not an English major, but I don't think you've got the right word,
 assuming some are greater and some are less than 35th.

Definition #5 from Merriam Webster's Dictionary (see below) defines
substantial as "being largely but not wholly that which is specified,"
implying the subject/object is 'less than' what it is being compared
to.  Since some of your objects are probably greater than 35', it
appears 'substantial' is not quite the word you're looking for.

Are you opposed to just using the phrase "approximately 35 feet?"

Main Entry: sub·stan·tial 
Pronunciation: s&b-'stan(t)-sh&l
Function: adjective
Date: 14th century
1 a : consisting of or relating to substance b : not imaginary or
illusory : REAL, TRUE c : IMPORTANT, ESSENTIAL
2 : ample to satisfy and nourish : FULL <a substantial meal>
3 a : possessed of means : WELL-TO-DO b : considerable in quantity :
significantly great <earned a substantial wage>
4 : firmly constructed : STURDY
5 : being largely but not wholly that which is specified <a
substantial lie>

Hi,

They are "substantially" the same length.

That said, there are many words which better fit the
idea you which to express. However, I take it that for
reasons of preexisting contractual terminology or for 
other reasons of legal significance you need to know about
the use of "substantially" in this context. 

I must disagree with the comment of respree-ga in that "not
wholly" means not entirely and has no imlications with regard
to "less than" as opposed to "greater than".

Best Regards,
Raisondetre-ga