If you're looking at a neighborhood shopping center with a major grocery anchor, how much in-line space is considered ideal? There is no one correct answer to your question as there are a lot of factors which will dictate what can be done (or should be done) with each specific site. Neighborhood grocery anchored centers are typically 75,000 - 125,000 sf in size. If you factor in most major grocers run 45 - 65,000 sf (some even a little larger), this is one factor that will help determine what might be done with the remaining, in addition to things like overall size of the site, how much is buildable, population density, parking ratio requirements, etc.
Also, let's say you have a major grocery store, but also have a very popular restaurant, such as a local pizza or Mexican place. Would those kinds of restaurants or other popular in-line tenants improve the grocery sales? For example, stopping at the grocery store while picking up laundry or the kids from a daycare? Sure, there's benefit that goes both ways between anchors and small shop tenants. However, at the end of the day, it's the anchor tenant that drives business to the center, and often anchors will dictate what types of tenants they will allow as co-tenants in the center and may even impose radius restrictions within so many feet of their door depending on the tenant use. There is one anchor tenant that comes to mind who will not allow restaurants, gyms, churches within 300 ft of their premises (within the center) as these tenants use up a lot of parking for extended periods of time.
If the grocery store is dated, how difficult is it to remodel? Can this be done in a way to minimize hassle to shoppers or is a complete rebuild more usual? Can't speak to the specifics on how best to handle other than I know remodels happen every so many years with national tenants to keep their stores looking current and modern. I have one center in particular that the anchor recently remodeled their store and did it in sections. At the same same, a smaller national tenant will be moving temporarily into the adjacent space while they do their store remodel.