Knowing Kṛṣṇa as He Is
55
primeval Lord, whose transcendental form is full of bliss, truth and substantiality and is thus full of the most dazzling splendor. Each of the limbs of that transcendental figure possesses, in Himself, the full-fledged functions of all the organs, and eternally sees, maintains and manifests the infinite universes, both spiritual and mundane.” (Bs. 5.30,32)
This form has nothing whatsoever to do with material forms. Impersonalists say, “Oh, you say Kṛṣṇa has a form. If so, how can you say that He is the Supreme? The impersonal Brahman is the Supreme, and the impersonal Brahman is formless.” But we have information from  Bhagavad-gītā  that Kṛṣṇa is the source of the impersonal Brahman.
brahmaṇo hi pratiṣṭhāham amṛtasyāvyayasya ca śāśvatasya ca dharmasya sukhasyaikāntikasya ca
“And I am the basis of the impersonal Brahman, which is immortal and imperishable, eternal, the constitutional position of ultimate happiness.” (Bg. 14.27)
Kṛṣṇa certainly has form, but His form, as stated before, is  sac-cid-ānanda-vigraha , eternal, full of bliss, and full of knowledge. The attributes of His transcendental body are summarized by Lord Brahmā in this way:
īśvaraḥ paramaḥ kṛṣṇaḥ sac-cid-ānanda-vigrahaḥ