The answer to this question depends on many factors and qualifiers. Therefore, I will answer the question and feel pretty confident that someone will be able to come up with something longer.
The longest two non-hyphenated words listed in the Central Sinama dictionary with no enclitic pronouns are:
pagsiningkulang* (15 letters, 13 phonemes) – sitting cross-legged.
panganggidgiran (15 letters, 13 phonemes) – a grindstone or grinding wheel.
Here are some of the problems with these two words. They are derivatives that come from shorter Sinama roots (singkulang and gidgid).
Pagsiningkulang has a prefix (pag-) and an infix (-in-) and panganggidgiran has a prefix (pang-) and suffix (-an).
Both have the ‘ng’ phoneme twice. This means if they were spelled using a different spelling system they could be shortened to 13 (pagsiniŋkulaŋ and paŋaŋgidgiran).
They could still be lengthened further with enclitic pronouns.
Timanin na batu panganggidgirantam.
Mbal kasulutan si ina’ ma pagsiningkulangtam ma riyata’ lamisan.
Both words now become 18 letters long.
* I am told that pagsiningkullang is the correct spelling/pronunciation. This is not how the word is represented in the dictionary. In that case it becomes tied for the longest non-hyphenated word in the Sinama language.
The following words in the dictionary are shorter in letters but longer phonemically:
kabuwattituhan (14 letters, 14 phonemes) – The present era or time.
pagbalbantahan (14 letters, 14 phonemes) – To relate to each other as enemies.
Neither of these words can take an enclitic pronoun.
Also, the Central Sinama – English Dictionary is not exhaustive and may be more complete for some dialects of Sinama over others. The longest non-hyphenated word in the dictionary phonemically is kabuwattituhan. The word kabuwattinaꞌanan immediately comes to mind. It is a dialect difference for the same word.
kabuwattinaꞌanan (16 letters, 16 phonemes) – The present era or time.
The longest non-derivative word in the Central Sinama – English Dictionary is:
kadjalangking (13 letters, 11 phonemes) – Scorpion (jalalangking in other dialects)
Equally long phonemically are the words:
kansinsilyu/gansinsilyu/kalsinsilyu (11 letters, 11 phonemes) – Men’s undershorts.
halahuwalam (11 letters, 11 phonemes) – An expression of complete ignorance of a matter.
I will venture into the topic of hyphenated words in Sinama where things become quite complicated. Hyphens occur with word derivations involving duplication and compound words.
A large amount of Sinama’s nouns and verbs can be duplicated. It is possible that an abnormally small scorpion or a toy scorpion could be called a kadjalangking-kadjalangking. That is a 27 letter word, but its not in the dictionary and I don’t know if that word has ever been spoken before.
Compound words can use hyphens.
There are several words in the Central Sinama – English Dictionary with two hyphens:
tabꞌllung-banggaꞌ-banggaꞌ (25 letters, 20 phonemes) – Thornback cowfish.
kalitan-tutungan-sꞌllang (24 letters, 20 phonemes) – Blacktip shark.
kalitan-kallang-kallang (23 letters, 19 phonemes) – Mako shark.
I’m not impressed with these words when compared with kabuwattinaꞌanan. I would argue that a three hyphened Sinama word should be two words. Usually a kind of creature and then a describing adjective.
Pagtungkellat-tungkellat is an impressive word albeit a derivative.
pagtungkellat-tungkellat (24 letters, 21 phonemes) – To rock back and forth like on a seesaw.
also impressive is
buwattingga-buwattingga (23 letters, 20 phonemes) – In any way whatsoever.
The duplication itself makes this a derivative but it is unaffixed.
Talꞌngnget-lꞌngnget is an impressive word:
talꞌngnget-lꞌngnget (19 letters, 14 phonemes) – Cicada.
I know of no Sinama word lꞌngnget, so this is presumably a non-derived hyphenated Sinama word.
sajumaꞌat-sajumaꞌat (19 letters, 19 phonemes) – Every Friday.
Which of these words do you think should count as Sinama’s longest word?
Can you come up with something longer?