Probably you are referring to MMS and not iMessage.
iMessage has no limit only MMS has a size limit of 1MB that too with some carrier. MMS is a carrier specific feature. Apple Inc does not provide MMS services.
MMS (Multimedia Messaging Service) does have some limitations on photo size. In most cases, MMS messages have a file size limit, and the maximum size for an MMS photo is typically around 1 MB (megabyte). This can translate to roughly a 1-megapixel photo if it is not heavily compressed.
The actual file size limit for MMS may vary depending on the carrier, the specific phone model, and the settings on the device. Some carriers and devices may have slightly higher or lower limits for MMS photo sizes. Additionally, some smartphones may automatically compress images sent via MMS to reduce their size and stay within the limits.
If you need to send higher resolution photos or larger files, it's often better to use alternative methods such as email or instant messaging platforms that support larger file attachments. Many messaging apps nowadays allow you to send higher quality images without such strict size limitations.