Comparison is Sindarin is a bit tricky. We have what appears to an intensive or superlative prefix ro- (< rau-) as in rovaed, which may mean “*very shapely” or “*most shapely” (PE17/147); this suffix seems to be the equivalent of Q. ar(i)- which elsewhere was described as “virtually superlative” (PE17/56). We also have hints of a comparative prefix in words like einior “elder”. Probably the initial element of this word is related to the Quenya intensive prefix an-, with the initial ein- developing via i-affection/intrusion from an- + iaur “old”. A Sindarin intensive an- might also be seen in names like Anfauglith “Gasping Dust”, with [N.] faug “thirsty” intensified to “gasping”.
The Noldorin form of this intensive prefix was described somewhat differently in The Etymologies of the 1930s. Here the intensive prefix is given as N. a- with “dynamic lengthening” (EtyAC/N) with two examples: afarch “very dry” (< appharkha) = a- + parch and angol “stench” (< aññol) = a- + ÑOL. At this conceptual stage, the consonant a- induced a lengthened initial consonant rather than nasal mutation, but in most cases the result is the same. For example: an+p- > amph- > aff- produces the same result as app- > apph- > aff-.
Conceptual Development: Both Gnomish and Early Noldorin had well developed systems of comparative suffixes. In the Gnomish Grammar of the 1910s, the suffixes were augmentative -odron/-oglon “*more” and diminutive -inthir “*less”, with superlative variants -onta “*most” and -inci “*least” (GG/16). In the Early Noldorin Grammar the comparative/superlative suffixes were -iad/-iant, often with modified adjective stems: beleg “great” → beliad “*greater”, beliant “*greatest” (PE13/125). Neither of these systems are compatible with Sindarin.
Neo-Sindarin: Thorsten Renk proposed a system of Sindarin comparatives in his book Pedin Edhellen: a Sindarin-Course (PESC/45), and I recommend using something similar. For the intensive, he recommends prefixing an- with nasal mutation of the following consonant, basically behaving like the plural definite article in, so an + tanc “firm” = athanc “very firm, firmer”. It would assimilate to following labials as usual, so an- + beleg “strong” = ammeleg “very strong, stronger”. Before adjectives with i, Renk recommends i-affection for the prefix, so that an- + cîr “new” = echir “very new, newer”. Before words beginning with consonantal i-, he recommends intrusion producing ein- (as in einior “elder”), so an- + iaun “wide” = einion “very wide, wider”.
Renk recommends using ro- as another type of intensive, but I think it is better to treat it as a superlative, as suggested by Fiona Jallings in her book, A Fan’s Guide to Neo-Sindarin (FGNS/160); this prefix induces soft mutation. Thus rodanc “firmest”, roveleg “strongest”, rogir “newest”. I would use this superlative more or less like an ordinary adjective, as in: Galadriel i edhel rohael “Galadriel (is) the wisest (ro- + sael) elf”.
For comparing two things, Renk suggested using [N.] athan “beyond” as in Aragorn beleg athan Boromir “Aragorn (is) strong beyond Boromir (= stronger than Boromir)”. This system is analogous to Quenya comparitives which likewise use a preposition meaning “beyond” (PE17/90). I think this comparison can be intensified by using an intensive prefix, hence: Aragorn ammeleg athan Boromir = “Aragorn (is) much stronger than Boromir”.
Renk’s system is largely guesswork, but it’s probably the best we can do until we get more information.