Because I'm not strong in prosody, I may do violence to this passage.
My translation:251 syllaba longa brevi subiecta vocatur Iambus,
252 pes citus: unde etiam trimetris adcrescere iussit
253 nomen iambeis, cum senos redderet ictus,
254 primus ad extremum similis sibi: non ita pridem,
A long syllable attached after a short is called an iamb
a quick-moving foot: accordingly it ordered the name of trimeter to be annexed to
the (concept of) the iambic, even though the trimeter calls for six beats (senos ictus),
each one (of these six ) the same.
Grammar calls
syllaba . . . brevi subiecta: subiecta modifies syllaba; brevi is dative complement of subiecta
iussit: implied subject is pes, or iambus
similis sibi: the antecedent of sibi is iambus, or pes
iambeis: dat. pl. of iambeus, an adjective used substantively, dative complement of adcrescere
adcrescere: s.v. accresco L&S: "To be added to by way of increase or augmentation, to be joined or annexed to..."
cum . . . redderet: I render this as cum concessive.
pes, iambus, ictus: As i understand, pes is the general term for poetic foot; iambus is the name for a particular kind of poetic foot, and ictus is the name for the stressed syllable in a poetic foot.
Remaining puzzles: My translation hardly explains why a line of six feet is called a trimeter. As I understand a Latin iambic trimeter, consists ideally of six iambs, three groups of two each. If you think of each pair as the unit of versification, then trimeter makes sense, but I couldn't find that idea in this selection. Besides that Horace seems to use pes and ictus interchangeably. I expect his audience could get this quickly with little contextual discussion, as I cannot.