This species is characterized by its shrub or tree habit, unusually robust, petiolate, obovate leaves, cymose inflorescences, pedicellate 6-merous flowers, shallowly lobed calyx limbs 4-8 mm long, funnelform corollas with the tube 32-34 mm long and the rounded lobes 2-3 mm long, relatively large anthers, and woody ellipsoid capsules up to 7 x 4 cm. The leaves are variable in size and form. They range up to 50 x 35 cm, and from broadly obovate to rather narrowly so or elliptic. This entire range of shape is shown on the duplicates of the type collection of Sickingia catappifolia. The leaf blades vary from glabrous to densely pilosulous or hirtellous on the lower surface, but the presence of this pubescence does not apppear to be correlated with any other features. The flowers are unusual in form, as noted below. The capsules are lenticellate at maturity, and unusual in Simira in their ellipsoid shape and apical beak portion, which is formed by the disk tissues. The capsule walls split into two hemispherical sections, but are then often unusual in Simira in having these walls split again to form four valves. Simira macrocrater dries yellowish brown with a weakly purplish cast, similarly to various other species of the genus, but does not show the characteristic reddish purple oxidation of the wood in stem cuts.

The flowers of Simira macrocrater change shape markedly during development, so plants in bud are difficult to associate with a partiuclar Rubiaceae genus. The corollas are tubular in bud, and the lobes are generally held erect on dried specimens. The aestivation of this species has been described as open (Barbosa & Peixoto, 1989), but the lobes do appear to be folded to cover the top of the bud on very young buds (e.g., Nee & Molina 58632); whether they spread later or the condition on dried specimens is an artifact is not clear. The corollas are yellowish green, and at anthesis they are yellowish green and reportedly fragrant ("with strong fragrance of jasmine and chlorine"; Nee 37677). At anthesis the corollas also expand, to elongate and spread laterally into a relatively large (for Simira) funnelform shape. The mature anthers are exserted and ca. 12 mm long. The stigmas are also exserted, and perhaps 7-8 mm long. The flower form and color suggest that this species may be pollinated by bats.

Simira macrocrater is variable and unusual in several features, and ranges into both deciduous and evergreen vegetation, and several names are syonoymized here. Simira williamsii was described based on one fruiting collecton with relatively narrow leaves, but falls within the variation now documented in Simira macrocrater and is synonymized here. Simira macrocrater is generally similar vegetatively to Simira ecuadorensis, which has an unusual, generally similar though smaller flower form.