QB Issue Resolution:

It will have a hard limit on the number of characters allowed. I think it is 11 but haven’t checked. However you may be referring to the size of it. The Invoice Templates layouts effect the print out but not the onscreen entry field size (they do effect whether a particular field shows up or not in data entry).

Resolution for Issue 'How can I increase the Invoice No. text box to include more characters?' available: Yes (Solved).
Source: Intuit Community forum.

How can I increase the Invoice No. text box to include more characters?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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