The 60-day notice and the request to enter your apartment for measurements are unrelated. My understanding of the law is that for a tenant who stayed on the property notice to vacate shall be delivered 60 days prior to the vacancy date. It would be 30 days if the tenant was living on the property for less than a year.
The notification about the measurements should have been delivered to you 24 hours prior to the time of entry. The landlord only have to notify you about the visit, they don't have to get your permission to enter the premises. The same process applies to landlord, or landlord's employee, entering your unit during the whole duration of the lease.
As others said - check with a lawyer if the landlord did everything correctly.... more