Hi @JakeCham,
According to documentation you should use maxlength property to restrict number of chars.
Here is very simple example that presents text filed with length limitation inside multi filed. I have tested it on AEM 6.5.11 but it should work on any other AEM 6.5 version.
<cq:dialog jcr:primaryType="nt:unstructured" jcr:title="Properties" sling:resourceType="cq/gui/components/authoring/dialog">
<content jcr:primaryType="nt:unstructured" sling:resourceType="granite/ui/components/coral/foundation/fixedcolumns">
<items jcr:primaryType="nt:unstructured">
<column jcr:primaryType="nt:unstructured" sling:resourceType="granite/ui/components/coral/foundation/container">
<items jcr:primaryType="nt:unstructured">
<multi jcr:primaryType="nt:unstructured" sling:resourceType="granite/ui/components/coral/foundation/form/multifield">
<field jcr:primaryType="nt:unstructured" maxlength="3" name="./value" sling:resourceType="granite/ui/components/coral/foundation/form/textfield"/>
</multi>
</items>
</column>
</items>
</content>
</cq:dialog>