"A big aeroplane"
There is an agreement. In this case, it's indicated by the lack of prefix on kubwa
Adjectives that go with class 9 or 10 nouns follow the "N-rule'. The following changes take place.
B > mb
W > mb
V > mv
D > nd
R > nd
L > nd
Z > nz
J > nj
G > ng
This may look random, but it's because Swahili has the following prenasalised phonemes: mb mv nd nz ng
Other consonants (ch, f, h, k, m, n, p, s, t, z) take no prefix. So that's why it's nzuri but kubwa. Adjectives starting with a vowel take ny- as the prefix, except for -ema which is njema instead of *nyema.
The only exception to this is when the adjective would, without a prefix, only be one syllable, such as -pya which takes a stressed m-.
And incidentally, if you say ndege mkubwa, that's placing it in the M-WA class, used for people and animals, so that actually means "a big bird".
Correct, except if it would otherwise only be one syllable. The adjective -ke for example, would be nke in this class. Adjectives, nouns and verbs have to have a syllable that is stressed, so one syllable words tend to be avoided, and in this case, the nasal prefix is stressed.
I don't think nke is commonly used though. Usually -a kike is used to mean female, or the noun jike.