The maximum size of the attachment in ALM 12.53 depends on the site parameter 'UPLOAD_ATTACH_MAX_SIZE'.
Below is the information about this site parameter.
Description: This parameter prevents the upload of attachments whose size is greater than the integer value specified for this parameter. The size is in kilobytes.
Note: This parameter does not affect uploads of attachments in integration tools. The default value is -1 (meaning, all attachments are uploaded
regardless of size).
Default value: -1
Min value: -1
Max value: 9223372036854775807
Introduced in release: QC 10.00
Introduced in patch: GA
Hope this helps you. If you have any questions or concerns, or need anything else regarding this, please let me know.