1. Create the main form, frmMain. The RecordSource property should be Table 1. Make sure that there is a control, txtSize, bound to the field [Table 1].[Size]
2. Create the subform, frmMain_Sub. Create a parameter query which will be the RecordSource of frmMain_Sub. The query SQL will look like something this:
SELECT [Table 2].* FROM [Table 2] WHERE ((([Table 2].[Size Field])>=([Forms]![frmMain]![txtSize]-100)) AND (([Table 2].[Size Field])<=([Forms]![frmMain]![txtSize]+100)));
Alternatively you could copy and paste the SQL string directly into the RecordSource property of frmMain_Sub.
3. Create a subForm control, subMain, on frmMain. Make sure the SourceObject is frmMain_Sub. If there is a relationship between Table 1 and Table 2 make sure that the values of LinkChildFields and LinkMasterFields are correct. Problems might occur if the size fields in Table 1 and Table 2 have the same name. Access tries to intelligently create LinkMasterField and LinkChildField values, and could create a link between the "Size" fields if your Tables are not related.
This scheme should mean that as you navigate records in frmMain, related and/or filtered records should appear in your frmMain_Sub.