There is generally no maximum word count for JMIR journals as we are an electronic journal and have no "hard" limits. Note, however, that we may have to charge extra copyediting fees if the paper (without data* tables) is over 5000 words (see What are your fees for "additional editorial/production work" and when are they applied?).
We suggest aiming for a maximum of 5000 words in the main body (without data* tables) and to move some information into a Multimedia Appendix if the manuscript is lengthy; the limiting factor is the readers' attention span. For example, detailed methods with formulas or lengthy tables are often better suited for a Multimedia Appendix (you may put an abridged table into the main manuscript, and move a detailed table with more columns into an Appendix).
If a table will appear more than 1.5 pages long in the final typeset PDF, you may be asked during copyediting OR proofreading to move the content to a Multimedia Appendix. This is generally non-negotiable. If a longer-than-usual table contains essential information to your paper, please condense it into an abridged version as detailed above prior to submitting.
Short papers are generally less than 1500 words.
*Tables consisting of majority text will be included in the word count as their contents do need to be addressed by the copyeditor.